Forensic science reflects multidisciplinary scientific approach to examining crime scenes and in gathering and examining evidence to be used in legal proceedings.

Forensic science techniques are also used to verify compliance with international treaties and resolutions regarding weapons production and use.

Forensic scientists are also involved in investigating accidents such as train or plane crashes to establish if they were accidental or a result of foul play. The techniques developed by forensic science are also used by the United States military to analyze the possibility of the presence of chemical weapons or high explosives, to test for propellant stabilizers, or to monitor compliance with international agreements regarding weapons of mass destruction.

The main areas used in forensic science are biology, chemistry, and medicine, although the science also includes the use of physics, computer science, geology, or psychology. Forensic scientists examine objects, substances (including blood or drug samples), chemicals (paints, explosives, toxins), tissue traces (hair, skin), or impressions (fingerprints or tidemarks) left at the crime scene. The majority of forensic scientists specialize in one area of science.

Basic philosophy

A basic principle of forensic science is that a criminal always brings something to the scene of a crime, and he or she always leaves something behind. The “something” left behind is the evidence that detectives and criminalists (people who make use of science to solve crimes) look for. It might be fingerprints, footprints, tooth marks, blood, semen, hair, fibers, broken glass, a knife or gun, a bullet, or something less tangible such as the nature of the wounds or bruises left on the victim’s body, which might indicate the nature of the weapon or the method of assault. Careful analysis of evidence left at the scene of a crime often can be used in establishing the guilt or innocence of someone on trial.

The analysis of the scene of a crime or accident involves obtaining a permanent record of the scene (forensic photography) and collection of evidence for further examination and comparison. Collected samples include biological (tissue samples such as skin, blood, semen, or hair), physical (fingerprints, shells, fragments of instruments or equipment, fibers, recorded voice messages, or computer discs) and chemical (samples of paint, cosmetics, solvents, or soil).

Most commonly, the evidence collected at the scene is subsequently processed in a forensic laboratory by scientists specializing in a particular area. Scientists identify, for example, fingerprints, chemical residues, fibers, hair, or DNA. However, miniaturization of equipment and the ability to perform most forensic analysis at the scene of crime results in more specialists being present in the field. Presence of more people at the scene of crime introduces a greater likelihood of introduction of contamination into the evidence. Moreover, multi-handling of a piece of evidence (for example, a murder weapon) is also likely to introduce traces of tissue or DNA not originating from the scene of a crime. Consequently, strict quality controls are imposed on collection, handling, and analysis of evidence to avoid contamination.

The ability to properly collect and process forensic samples can affect the ability of the prosecution to prove their case during a trial. The presence of chemical traces or DNA on a piece of debris is also crucial in establishing the chain of events leading to a crime or accident.

Physical evidence usually refers to objects found at the scene of a crime. Physical evidence may include all sorts of prints such as fingerprints, footprints, handprints, tidemarks, cut marks, tool marks, etc. Analysis of some physical evidence is conducted by making impressions in plaster, taking images of marks, or lifting the fingerprints from objects encountered. These serve later as a comparison to identify, for example, a vehicle that was parked at the scene, a person that was present, a type of manufacturing method used to create a tool, or a method used to break in a building or harm a victim.

Biological traces are collected not only from the scene of crime and a deceased person, but also from surviving victims and suspects. Most commonly, samples obtained are blood, hair, and semen. DNA can be extracted from any of these samples and used for comparative analysis.

DNA is the most accurate means of identification. Victims of crashes or fires are often unrecognizable, but if adequate DNA can be isolated a person can be positively identified if a sample of their DNA or their family’s DNA is taken for comparison. Such methods are being used in the identification of the remains in Yugoslav war victims, the World Trade Center terrorist attack victims, the 2002 Bali bombing victims, and victims of the 2004 Indian Ocean Tsunami.

Biological traces come from bloodstains, saliva samples (from cigarette buts or chewing gum) and tissue samples, such as skin, nails, or hair. Samples are processed to isolate the DNA and establish the origin of the samples. Samples must first be identified as human, animal, or plant before further investigation proceeds. For some applications, such as customs and quarantine, traces of animal and plant tissue have to be identified to the level of the species, as transport of some species is prohibited. A presence of a particular species can also prove that a suspect or victim visited a particular area. In cases of national security, samples are tested for the presence of pathogens and toxins, and the latter are also analyzed chemically.

History

Archimedes (287–212 BC), who proved that his king’s crown was not pure gold by measuring its density, was perhaps the world’s first forensic scientist. However, it was Sir Arthur Conan Doyle’s (1859–1930) fictional stories of Sherlock Holmes, written in the late nineteenth century, that first anticipated the use of science in solving crimes in the twentieth century. At about the same time, Sir Francis Galton’s (1822–1911) studies revealed that fingerprints are unique and do not change with age. As early as 1858, William Herschel (1833–1918), a British official in India, used imprints of inked fingers and hands as signatures on documents for people who could not write. Unknown to Herschel, contracts in Japan had been sealed by using a thumb or fingerprint for centuries.

During the 1890s, Scotland Yard, headquarters for the metropolitan police of London, began to use a system developed by a French police official named Alphonse Bertillon (1853–1914). The Bertillon system consisted of a photograph and 11 body measurements that included dimensions of the head, length of arms, legs, feet, hands, and so on. Bertillon claimed that the likelihood of two people having the same measurements for all 11 traits was less than one in 250 million. In 1894, fingerprints, which were easier to use and more unique (even identical twins have different fingerprints), were added to the Bertillon system.

Edmond Locard (1877–1966), a French criminalist, established the first laboratory dedicated to crime analysis in 1910. A decade later, crime labs had been established throughout Europe. The first crime lab in the United States was opened in Los Angeles in 1923, but it was 1932 before the Federal Crime Laboratory was established by the Federal Bureau of Investigation (FBI) under the direction of J. Edgar Hoover (1895–1972). Today, there are about 400 crime labs and nearly 40,000 people involved in forensic science in the United States alone.

Methodologies and areas of application

A number of analytical methods are used by forensic laboratories to analyze evidence from a crime scene. Methods vary, depending on the type of evidence analyzed and information extracted from the traces found. If a type of evidence is encountered for the first time, a new method is developed.

Fingerprints

Although fingerprints have been used by crime investigators for more than a century, they remain one of the most sought after pieces of evidence. All human beings are born with a characteristic set of ridges on our fingertips. The ridges, which are rich in sweat pores, form a pattern that remains fixed for life. Even if the skin is removed, the same pattern will be evident when the skin regenerates. Some of the typical patterns found in fingerprints are arches, loops, and whorls.

Oils from sweat glands collect on these ridges. When we touch something, a small amount of the oils and other materials on our fingers are left on the surface of the object we touched. The pattern left by these substances, which collect along the ridges on our fingers, make up the fingerprints that police look for at the scene of a crime. It is the unique pattern made by these ridges that motivate police to record people’s fingerprints. To take someone’s fingerprints, the ends of the person’s fingers are first covered with ink. The fingers are then rolled, one at a time, on a smooth surface to make an imprint that can be preserved. Fingerprints collected as evidence can be compared with fingerprints on file or taken from a suspect.

Everyone entering military service, the merchant marine, and many other organizations are fingerprinted. The prints are there to serve as an aid in identification should that person be killed or seriously injured. The FBI maintains a fingerprint library with patterns taken from more than 10% of the entire United States population. Each year the FBI responds to thousands of requests to compare samples collected as evidence with those on file at their library. The process of comparison has been improved in terms of speed and effectiveness in recent years by the development of automated fingerprint identification systems (AFIS) that allows police departments with computer access to search the collection.

Many fingerprints found at crime scenes are not visible. These latent fingerprints, which are often incomplete, are obtained in various ways. The oldest and most frequently used method is to use a powder, such as ninhydrin, to dust the surface. The powder sticks to the oily substances on the print making the pattern visible. The print can then be photographed and lifted off the surface by using a tape to which the powder adheres. To search for fingerprints on porous materials such as paper, forensic technicians use fumes of iodine or cyanoacrylate. These fumes readily collect on the oils in the print pattern and can be photographed. Since 1978, argon lasers have also been used to view latent fingerprints. When illuminated by light from an argon laser, a latent print is often quite visible. Visibility under laser light can be enhanced by first dusting the print with a fluorescent fingerprint powder.

Fingerprints are not the only incriminating patterns that a criminal may leave behind. Lip prints are frequently found on glasses. Footprints and the soil left on the print may match those found in a search of an accused person’s premises. Tire tracks, bite marks, toe prints, and prints left by bare feet may also provide useful evidence. In cases where the identity of a victim is difficult because of tissuedecomposition or death caused by explosions or extremely forceful collisions, a victim’s teeth may be used for comparison with the dental records of missing people.

Genetic fingerprints

The nuclei within our cells contain coiled, threadlike bodies called chromosomes. Chromosomes are paired, one member of each pair came from your father; the other one from your mother. Chromosomes are made of deoxyribonucleic acid, often called DNA. It is DNA that carries the “blueprint” (genes) from which “building orders” are obtained to direct the growth, maintenance, and activities that go on within our bodies.

Except for identical twins, no two people have the same DNA. However, we all belong to the same species; consequently, large strands of DNA are the same in all of us. The segments that are different among us are often referred to as junk DNA by biologists. It is these unique strands of DNA that are used by forensic scientists. Strands of DNA can be extracted from cells and “cut” into shorter sections using enzymes. Through chemical techniques involving elec-trophoresis, radioactive DNA, and x rays, a characteristic pattern can be established-the so-called genetic fingerprint. Because different people have different junk DNA, the prints obtained from different people will vary considerably; however, two samples from the same person will be identical. If there is a match between DNA extracted from semen found on the body of a rape victim and the DNA obtained from a rape suspect’s blood, the match is very convincing evidence-evidence that may well lead to a conviction or possibly a confession.

Although genetic fingerprinting can provide incriminating evidence, DNA analysis is not always possible because the amount of DNA extracted may not be sufficient for testing. Furthermore, there has been considerable controversy about the use of DNA, the statistical nature of the evidence it offers, and the validity of the testing.

Genetic fingerprinting is not limited to DNA obtained from humans. In Arizona, a homicide detective found two seed pods from a paloverde tree in the bed of a pickup truck owned by a man accused of murdering a young woman and disposing of her body. The accused man admitted giving the woman a ride in his truck but denied ever having been near the factory where her body was found. The detective, after noting a scrape on a paloverde tree near the factory, surmised that it was caused by the accused man’s truck. Using RAPD (randomly amplified polymorphic DNA) markers—a technique developed by Du Pont scientists—forensic scientists were able to show that the seed pods found in the truck must have come from the scraped tree at the factory.

Blood evidence

Long before DNA was recognized as the “ink” in the blueprints of life, blood samples were collected and analyzed in crime labs. Most tests used to tentatively identify a material as blood are based on the fact that peroxidase, an enzyme found in blood, acts as a catalyst for the reagent added to the blood and forms a characteristic color. For example, when benzidine is added to a solution made from dried blood and water, the solution turns blue. If phenolphthalein is the reagent, the solution turns pink. More specific tests are then applied to determine if the blood is human.

The evidence available through blood typing is not as convincing as genetic fingerprinting, but it can readily prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups–A, B, AB, or O. These blood groups are based on genetically determined antigens (A and/or B) that may be attached to the red blood cells. These antigens are either present or absent in blood. By adding specific antibodies (anti-A or anti-B) the presence or absence of the A and B antigens can be determined. If the blood cells carry the A antigen, they will clump together in the presence of the anti-A antibody. Similarly, red blood cells carrying the B antigen will clump when the anti-B antibody is added. Type A blood contains the A antigen; type B blood carries the B antigen; type AB blood carries both antigens; and type O blood, the most common, carries neither antigen. To determine the blood type of a blood sample, antibodies of each type are added to separate samples of the blood. The results, which are summarized in the table, indicate the blood type of the sample.

Table 1. (Thomson Gale.)
Testing for blood type
A+ indicates that the blood cells clump and, therefore, contain the antigen specific for the antibody added. A2indicates there is no clumping and that the blood lacks the antigen specific for the antibody added.
Antibody added to sample		Results of test indicates blood type to be
anti-A	anti-B
—	—	O
+	—	A
—	+	B
+	+	AB

If a person accused of a homicide has type AB blood and it matches the type found at the crime scene of a victim, the evidence for guilt is more convincing than if a match was found for type O blood. The reason is that only 4% of the population has type AB blood. The percentages vary somewhat with race. Among Caucasians, 45% have type O, 40% have type A, and 11 % have type B. African Americans are more likely to be type O or B and less likely to have type A blood.

When blood dries, the red blood cells split open. The open cells make identification of blood type trickier because the clumping of cell fragments rather than whole red blood cells is more difficult to see. Since the antigens of many blood-group types are unstable when dried, the FBI routinely tests for only the ABO, Rhesus (Rh), and Lewis (Le) blood-group antigens. Were these blood groups the only ones that could be identified from blood evidence, the tests would not be very useful except for proving the innocence of a suspect whose blood type does not match the blood found at a crime scene. Fortunately, forensic scientists are able to identify many blood proteins and enzymes in dried blood samples. These substances are also genetic markers, and identifying a number of them, particularly if they are rare, can be statistically significant in establishing the probability of a suspect’s guilt. For example, if a suspect’s ABO blood type matches the type O blood found at the crime scene, the evidence is not very convincing because 45% of the population has type O blood. However, if there is a certain match of two blood proteins (and no mismatches) known to be inherited on different chromosomes that appear respectively in 10% and 6% of the population, then the evidence is more convincing. It suggests that only 0.45 x 0.10 x 0.06 = 0.0027 or 0.27% of the population could be guilty. If the accused person happens to have several rarely found blood factors, then the evidence can be even more convincing.

Ballistics and tool markings

Ballistic analysis has been an important part of the work performed in crime labs. Comparison microscopes, which make it possible to simultaneously view and compare two bullets, are an important tool for forensic scientists. When a bullet is fired, it moves along a spiral groove in the gun barrel. It is this groove that makes the bullet spin so that it will follow a straight path much like that of a spiraling football. The striations or markings on the bullet made by the groove and the marks left by the firing pin are unique and can be used to identify the gun used to fire any bullets found at the scene of a homicide. Similarly, tool marks, which are often left by burglars who pry open doors or windows, can serve as useful evidence if comparisons can be made with tools associated with a person accused of the crime. Particularly incriminating are jigsaw matches-pieces of a tool left behind that can be shown to match pieces missing from a tool in the possession of the accused.

In the event that bullets have been shattered making microscopic comparisons impossible, the fragments may be analyzed by using neutron activation analysis. Such analysis involves bombarding the sample with neutrons to make the atoms radioactive. The gamma rays emitted by the sample are then scanned and compared with known samples to determine the concentration of different metals in the bullet-lead. The technique can be used to compare the evidence or sample with bullet-lead associated with the accused.

Autopsies

Pathologists and forensic anthropologists play a very important part in forensic examination. They are able to determine the cause of death by examining marks on the bone(s), skin (gunshot wounds), and other body surfaces for external trauma. They can also determine a cause of death by toxicological analysis of blood and tissues.

Autopsies can often establish the cause and approximate time of death. Cuts, scrapes, punctures, and rope marks may help to establish the cause of death. A drowning victim will have soggy lungs, water in the stomach, and blood diluted with water in the left side of the heart. A person who was not breathing when he or she entered the water will have undiluted blood in the heart. Bodies examined shortly after the time of death may have stiff jaws and limbs. Such stiffness, or rigor mortis, is evident about ten hours after death, but disappears after about a day when the tissues begin to decay at normal temperatures. Each case is different, of course, and a skillful coroner can often discover evidence that the killer never suspected he or she had left behind.

Forensic chemistry

Forensic chemistry performs qualitative and quantitative analysis of chemicals found on people, various objects, or in solutions. The chemical analysis is the most varied from all the forensic disciplines. Chemists analyze drugs as well as paints, remnants of explosives, fire debris, gunshot residues, fibers, and soil samples. They can also test for a presence of radioactive substances (nuclear weapons), toxic chemicals (chemical weapons), and biological toxins (biological weapons). Forensic chemists can also be called on in a case of environmental pollution to test the compounds and trace their origin.

The examination of chemical traces often requires very sensitive chromatographic techniques or mass spectrometric analysis. The four major types of chromatographic methods used are: thin layer chromatography (TLC) to separate inks and other chemicals; atomic absorption chromatography for analysis of heavy metals; gas chromatography (GC); and liquid chromatography (HPLC). GC is most widely used in identification of explosives, accelerators, propellants, and drugs or chemicals involved in chemical weapon production, while liquid chromatography (HPLC) is used for detection of minute amounts of compounds in complex mixtures. These methods rely on separation of the molecules based on their ability to travel in a solvent (TLC) or to adhere to adsorbent filling the chromatography column. By collecting all of the fractions and comparing the observed pattern to standards, scientists are able to identify the composition of even the most complex mixtures.

New laboratory instruments are able to identify nearly every element present in a sample. Because the composition of alloys used in production of steel instruments, wires, or bullet casings is different between various producers, it is possible to identify a source of the product.

In some cases chromatography alone is not an adequate method for identification. It is then combined with another method to separate the compounds even further and results in greater sensitivity. One such method is mass spectrometry (MS). A mass spectrometer uses high voltage to produce charged ions. Gaseous ions or isotopes are then separated in a magnetic field according to their masses. A combined GC-MS instrument has a very high sensitivity and can analyze samples present at concentrations of one part-per-billion.

As some samples are difficult to analyze with MS alone, a laser vaporization method (imaging laserablation mass spectroscopy) was developed to produce small amounts of chemicals from solid materials (fabrics, hair, fibers, soil, glass) for MS analysis. Such analysis can examine hair samples for presence of drugs or chemicals. Due to its high sensitivity, the method is of particular use in monitoring areas and people suspected of production of chemical, biological, or nuclear weapons, or narcotics producers.

While charcoal sticks are still in use for fire investigations, a new technology of solid-phase microextraction (SPME) was developed to collect even more chemicals and does not require any solvent for further analysis. The method relies on the use of sticks similar to charcoal, but coated with various polymers for collecting different chemicals (chemical warfare agents, explosives, or drugs). Collected samples are analyzed immediately in the field by GC.

Arson investigation

The identification of fire accelerants such as kerosene or gasoline is of great importance for determining the cause of a fire. Debris collected from a fire must be packed in tight, secure containers, as the compounds to be analyzed are often volatile. An improper transport of such debris would result in no detection of important traces. One of the methods used for this analysis involves the use of charcoal strips. The chemicals from the debris are absorbed onto the strip and subsequently dissolved in a solvent before analysis. This analysis allows scientists to determine the hydrocarbon content of the samples and identify the type of fire accelerator used.

Document examination

An examination of documents found at the scene or related to the crime is often an integral part of forensic analysis. Such examination is often able to establish not only the author but, more importantly, identify any alterations that have taken place. Specialists are also able to recover text from documents damaged by accident or on purpose.

The crime lab: a fusion of physics, chemistry, and molecular biology

Modern crime labs are equipped with various expensive analytical devices usually associated with research conducted by chemists and physicists. Scanning electron microscopes are used to magnify surfaces by as much as a factor of 200,000. Because the material being scanned emits x rays as well as secondary electrons in response to the electrons used in the scanning process, the microscope can be used together with an x ray micro analyzer to identify elements in the surface being scanned. The technique has been particularly successful in detecting the presence of residues left when a gun is fired.

The mass spectrometer and the gas chromatograph have been particularly effective in separating the components in illegal drugs, identifying them, and providing the data needed to track down their source and origin. Thin layer chromatography (TLC) has proved useful in identifying colored fibers. Although many fibers may appear identical under the microscope, they can often be distinguished by separating the component dyes used in coloring the fabric. Fusion microscopy—using changes in birefringence with temperature—has also proved useful in identifying and comparing synthetic fibers found at crime scenes. In addition to using such physical properties as density, dispersion, and refractive index to match and identify glass samples, the plasmaemis-sionspectroscope has proven helpful in analyzing the component elements in glass as well as distinguishing among various types of glass found in windows, bottles, and windshields.

In 2002, forensic science specialists played an integral role in the tracking and eventual identification of evidence (e.g. similarities in ballistics, psychological profiles, etc.) that allowed investigators link a nationwide a string of crimes that culminated in several snipers attacks in the Washington-Virginia-Maryland area.

Biological samples are most commonly analyzed by polymerase chain reaction (PCR). The results of PCR are then visualized by gel electrophoresis. Forensic scientists tracing the source of a biological attack could use the new hybridization or PCR-based methods of DNA analysis. Biological and chemical analysis of samples can also identify toxins found.

Forensic science and security issues

A growing area of forensic analysis is monitoring non-proliferation of weapons of mass destruction, analysis of possible terrorist attacks, or breaches of security. The nature of samples analyzed is wide, but slightly different from a criminal investigation. In addition to the already-described samples, forensic scientists who gather evidence of weapons of mass destruction collect swabs from objects, water, and plant material to test for the presence of radioactive isotopes, toxins, or poisons, as well as chemicals that can be used in production of chemical weapons. The main difference from the more common forensic

KEY TERMS

Birefringence —Splitting of light into two separate beams in which the light travels at different speeds.

Gas chromatograph —A device that separates and analyzes a mixture of compounds in gaseous form.

Mass spectrometer —A device that uses a magnetic field to separate ions according to their mass and charge.

Polymorphic —Distinct inherited traits that are different among members of the same species. Blood groups, for example, are polymorphic, but weight is not.

Scanning electron microscope —A device that emits a focused beam of electrons to scan the surface of a sample. Secondary electrons released from the sample are used to produce a signal on a cathode ray tube where the enlarged image is viewed.

investigation is the amount of chemicals present in a sample. Samples taken from the scene of suspected chemical or biological weapons often contain minute amounts of chemicals and require very sensitive and accurate instruments for analysis.

Forensic imaging

Imaging used by forensic scientists can be as simple as a light microscope, or can involve an electron microscope, absorption in ultraviolet to visible range, color analysis, or fluorescence analysis. Image analysis is used not only in cases of biological samples, but also for analysis of paints, fibers, hair, gunshot residue, or other chemicals. Image analysis is often essential for an interpretation of physical evidence. Specialists often enhance photographs to visualize small details essential in forensic analysis. Image analysis is also used to identify details from surveillance cameras.

Forensics in the virtual age

A new and emerging area of forensic science involves the reconstruction of computer data. High speed and large memory capacity computers also allow for what forensic investigators term as “virtual criminality,” the ability of computer animation to recreate crime scenes and or predict terrorism scenarios.

The identification of people can be performed by fingerprint analysis or DNA analysis. When none of these methods is viable, facial reconstruction (a combination of computer technology driven extrapolation and artistic rendering can be used instead to generate a person’s image. TV and newspapers then circulate the image for identification.

Forensic science and popular culture

Detective stories have long captures popular fascination but the role and impact of modern forensic sciences came to the forefront of public attention during the highly publicized and televised O.J. Simpson murder trial. Lawyers for both sides offered a wide variety of forensic evidence—and disputed the validity of opposing forensic evidence—before the controversial acquittal of Simpson.

Forensic science also continues to capture the imagination and fascination of popular culture. As of December 2006, several of the top-ranked televisions shows in the United States and Europe were dramas (fictional adaptations) involving the actions of forensic investigators and the techniques of forensic science.

Resources

BOOKS

Butler, John M.Forensic DNA Typing: the Biology and Technology Behind STR Markers. Boston: Academic Press, 2001.

Lee, Henry C., Gill, Charles D. Cracking Cases: The Science of Solving Crimes. Amherst, NY: Prometheus Books, 2002.

Nordby, Jon J. Dead Reckoning: The Art of Forensic Detection. Boca Raton, FL: CRC Press, 2000.

Sachs, Jessica S. Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death. An Exploration of the Haunting Science of Forensic Ecology. Cambridge, MA: Perseus Publishing, 2001.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. New York: Prentice-Hall 2000.

The Gale Encyclopedia of Science

Forensic Science

views updated May 29 2018

Forensic science

Forensic science reflects multidisciplinary scientific approach to examining crime scenes and in examining evidence to be used in legal proceedings. Forensic science techniques are also used to verify compliance with international treaties and resolutions regarding weapons production and use.

Forensic science techniques incorporate techniques and principles of biology , chemistry , medicine, physics , computer science, geology , and psychology .

Forensic science is the application of science to matters of law. Both defense and prosecuting attorneys sometimes use information gleaned by forensic scientists in attempting to prove the innocence or guilt of a person accused of a crime.

A basic principle of forensic science is that a criminal always brings something to the scene of a crime, and he or she always leaves something behind. The "some-thing" left behind is the evidence that detectives and criminalists (people who make use of science to solve crimes) look for. It might be fingerprints, footprints, tooth marks, blood , semen, hair, fibers, broken glass , a knife or gun, a bullet, or something less tangible such as the nature of the wounds or bruises left on the victim's body, which might indicate the nature of the weapon or the method of assault. Careful analysis of evidence left at the scene of a crime often can be used in establishing the guilt or innocence of someone on trial.

History

Archimedes, who proved that his king's crown was not pure gold by measuring its density , was perhaps the world's first forensic scientist. However, it was Sir Arthur Conan Doyle's fictional stories of Sherlock Holmes, written in the late nineteenth century, that first anticipated the use of science in solving crimes in the twentieth century. At about the same time, Sir Francis Galton's studies revealed that fingerprints are unique and do not change with age. As early as 1858, William Herschel, a British official in India, used imprints of inked fingers and hands as signatures on documents for people who could not write. Unknown to Herschel, contracts in Japan had been sealed by using a thumb or fingerprint for centuries.

During the 1890s, Scotland Yard, headquarters for the metropolitan police of London, began to use a system developed by a French police official named Alphonse Bertillon. The Bertillon system consisted of a photograph and 11 body measurements that included dimensions of the head, length of arms, legs, feet, hands, and so on. Bertillon claimed that the likelihood of two people having the same measurements for all 11 traits was less than one in 250 million. In 1894, fingerprints, which were easier to use and more unique (even identical twins have different fingerprints), were added to the Bertillon system.

Edmond Locard, a French criminalist, established the first laboratory dedicated to crime analysis in 1910. A decade later, crime labs had been established throughout Europe . The first crime lab in the United States was opened in Los Angeles in 1923, but it was 1932 before the Federal Crime Laboratory was established by the Federal Bureau of Investigation (FBI) under the direction of J. Edgar Hoover. Today, there are about 400 crime labs and nearly 40,000 people involved in forensic science in the United States alone.

Fingerprints

Although fingerprints have been used by crime investigators for more than a century, they remain one of the most sought after pieces of evidence. All human beings are born with a characteristic set of ridges on our fingertips. The ridges, which are rich in sweat pores, form a pattern that remains fixed for life. Even if the skin is removed, the same pattern will be evident when the skin regenerates. Some of the typical patterns found in fingerprints are arches, loops, and whorls.

Oils from sweat glands collect on these ridges. When we touch something, a small amount of the oils and other materials on our fingers are left on the surface of the object we touched. The pattern left by these substances, which collect along the ridges on our fingers, make up the fingerprints that police look for at the scene of a crime. It is the unique pattern made by these ridges that motivate police to record people's fingerprints. To take someone's fingerprints, the ends of the person's fingers are first covered with ink. The fingers are then rolled, one at a time, on a smooth surface to make an imprint that can be preserved. Fingerprints collected as evidence can be compared with fingerprints on file or taken from a suspect.

Everyone entering military service, the merchant marine, and many other organizations are fingerprinted. The prints are there to serve as an aid in identification should that person be killed or seriously injured. The FBI maintains a fingerprint library with patterns taken from more than 10% of the entire United States population. Each year the FBI responds to thousands of requests to compare samples collected as evidence with those on file at their library. The process of comparison has been improved in terms of speed and effectiveness in recent years by the development of automated fingerprint identification systems (AFIS) that allows police departments with computer access to search the collection.

Many fingerprints found at crime scenes are not visible. These latent fingerprints, which are often incomplete, are obtained in various ways. The oldest and most frequently used method is to use a powder such as ninhydrin to dust the surface. The powder sticks to the oily substances on the print making the pattern visible. The print can then be photographed and lifted off the surface by using a tape to which the powder adheres. To search for fingerprints on porous materials such as paper , forensic technicians use fumes of iodine or cyanoacry-late. These fumes readily collect on the oils in the print pattern and can be photographed. Since 1978, argon lasers have also been used to view latent fingerprints. When illuminated by light from an argon laser , a latent print is often quite visible. Visibility under laser light can be enhanced by first dusting the print with a fluorescent fingerprint powder.

Fingerprints are not the only incriminating patterns that a criminal may leave behind. Lip prints are frequently found on glasses. Footprints and the soil left on the print may match those found in a search of an accused person's premises. Tire tracks, bite marks, toe prints, and prints left by bare feet may also provide useful evidence. In cases where the identity of a victim is difficult because of tissue decomposition or death caused by explosions or extremely forceful collisions, a victim's teeth may be used for comparison with the dental records of missing people.

Genetic fingerprints

The nuclei within our cells contain coiled, thread-like bodies called chromosomes. Chromosomes are paired, one member of each pair came from your father; the other one from your mother. Chromosomes are made of deoxyribonucleic acid, often called DNA. It is DNA that carries the "blueprint" (genes) from which "building orders" are obtained to direct the growth, maintenance, and activities that go on within our bodies.

Except for identical twins, no two people have the same DNA. However, we all belong to the same species ; consequently, large strands of DNA are the same in all of us. The segments that are different among us are often referred to as junk DNA by biologists. It is these unique strands of DNA that are used by forensic scientists. Strands of DNA can be extracted from cells and "cut" into shorter sections using enzymes. Through chemical techniques involving electrophoresis , radioactive DNA, and x rays , a characteristic pattern can be establishedthe so-called genetic fingerprint. Because different people have different junk DNA, the prints obtained from different people will vary considerably; however, two samples from the same person will be identical. If there is a match between DNA extracted from semen found on the body of a rape victim and the DNA obtained from a rape suspect's blood, the match is very convincing evidence-evidence that may well lead to a conviction or possibly a confession.

Although genetic fingerprinting can provide incriminating evidence, DNA analysis is not always possible because the amount of DNA extracted may not be sufficient for testing. Furthermore, there has been considerable controversy about the use of DNA, the statistical nature of the evidence it offers, and the validity of the testing.

Genetic fingerprinting is not limited to DNA obtained from humans. In Arizona, a homicide detective found two seed pods from a paloverde tree in the bed of a pickup truck owned by a man accused of murdering a young woman and disposing of her body. The accused man admitted giving the woman a ride in his truck but denied ever having been near the factory where her body was found. The detective, after noting a scrape on a paloverde tree near the factory, surmised that it was caused by the accused man's truck. Using RAPD (Randomly Amplified Polymorphic DNA) markers—a technique developed by Du Pont scientists—forensic scientists were able to show that the seed pods found in the truck must have come from the scraped tree at the factory.

DNA analysis is a relatively new tool for forensic scientists, but already it has been used to free a number of people who were unjustly sent to prison for crimes that genetic fingerprinting has shown they could not have committed. Despite its success in freeing victims who were unfairly convicted, many defense lawyers claim prosecutors have overestimated the value of DNA testing in identifying defendants. They argue that because analysis of DNA molecules involves only a fraction of the DNA, a match does not establish guilt, only a probability of guilt. They also contend that there is a lack of quality control standards among laboratories, most of them private, where DNA testing is conducted. Lack of such controls, they argue, leads to so many errors in testing as to invalidate any statistical evidence. Many law officials argue that DNA analysis can provide probabilities that establish guilt beyond reasonable doubt.

Evidence and tools used in forensic science

Long before DNA was recognized as the "ink" in the blueprints of life, blood samples were collected and analyzed in crime labs. Most tests used to tentatively identify a material as blood are based on the fact that peroxidase, an enzyme found in blood, acts as a catalyst for the reagent added to the blood and forms a characteristic color . For example, when benzidine is added to a solution made from dried blood and water , the solution turns blue. If phenolphthalein is the reagent, the solution turns pink. More specific tests are then applied to determine if the blood is human.

The evidence available through blood typing is not as convincing as genetic fingerprinting, but it can readily prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups–A, B, AB, or O. These blood groups are based on genetically determined antigens (A and/or B) that may be attached to the red blood cells. These antigens are either present or absent in blood. By adding specific antibodies (anti-A or anti-B) the presence or absence of the A and B antigens can be determined. If the blood cells carry the A antigen, they will clump together in the presence of the anti-A antibody. Similarly, red blood cells carrying the B antigen will clump when the anti-B antibody is added. Type A blood contains the A antigen; type B blood carries the B antigen; type AB blood carries both antigens; and type O blood, the most common, carries neither antigen. To determine the blood type of a blood sample , antibodies of each type are added to separate samples of the blood. The results,

**Table: Testing for blood type.** **A + indicates that the blood cells clump and, therefore, contain the antigen specific for the antibody added.** **A - indicates there is no clumping and that the blood lacks the antigen specific for the antibody added.**
Antibody added to sample		Results of test indicates blood type to be
anti-A	anti-B
—	—	O
+	—	A
—	+	B
+	+	AB

which are summarized in the table, indicate the blood type of the sample.

If a person accused of a homicide has type AB blood and it matches the type found at the crime scene of a victim, the evidence for guilt is more convincing than if a match was found for type O blood. The reason is that only 4% of the population has type AB blood. The percentages vary somewhat with race. Among Caucasians, 45% have type O, 40% have type A, and 11% have type B. African Americans are more likely to be type O or B and less likely to have type A blood.

When blood dries, the red blood cells split open. The open cells make identification of blood type trickier because the clumping of cell fragments rather than whole red blood cells is more difficult to see. Since the antigens of many blood-group types are unstable when dried, the FBI routinely tests for only the ABO, Rhesus (Rh), and Lewis (Le) blood-group antigens. Were these blood groups the only ones that could be identified from blood evidence, the tests would not be very useful except for proving the innocence of a suspect whose blood type does not match the blood found at a crime scene. Fortunately, forensic scientists are able to identify many blood proteins and enzymes in dried blood samples. These substances are also genetic markers, and identifying a number of them, particularly if they are rare, can be statistically significant in establishing the probability of a suspect's guilt. For example, if a suspect's ABO blood type matches the type O blood found at the crime scene, the evidence is not very convincing because 45% of the population has type O blood. However, if there is a certain match of two blood proteins (and no mismatches) known to be inherited on different chromosomes that appear respectively in 10% and 6% of the population, then the evidence is more convincing. It suggests that only 0.45 X 0.10 X 0.06 = 0.0027 or 0.27% of the population could be guilty. If the accused person happens to have several rarely found blood factors, then the evidence can be even more convincing.

Since handguns are used in half the homicides committed in the United States and more than 60% of all homicides are caused by guns, it is not surprising that ballistic analysis has been an important part of the work performed in crime labs. Comparison microscopes, which make it possible to simultaneously view and compare two bullets, are an important tool for forensic scientists. When a bullet is fired, it moves along a spiral groove in the gun barrel. It is this groove that makes the bullet spin so that it will follow a straight path much like that of a spiraling football. The striations or markings on the bullet made by the groove and the marks left by the firing pin are unique and can be used to identify the gun used to fire any bullets found at the scene of a homicide. Similarly, tool marks, which are often left by burglars who pry open doors or windows, can serve as useful evidence if comparisons can be made with tools associated with a person accused of the crime. Particularly incriminating are jigsaw matches-pieces of a tool left behind that can be shown to match pieces missing from a tool in the possession of the accused.

In the event that bullets have been shattered making microscopic comparisons impossible, the fragments may be analyzed by using neutron activation analysis . Such analysis involves bombarding the sample with neutrons to make the atoms radioactive. The gamma rays emitted by the sample are then scanned and compared with known samples to determine the concentration of different metals in the bullet-lead. The technique can be used to compare the evidence or sample with bullet-lead associated with the accused.

Autopsies can often establish the cause and approximate time of death. Cuts, scrapes, punctures, and rope marks may help to establish the cause of death. A drowning victim will have soggy lungs, water in the stomach, and blood diluted with water in the left side of the heart . A person who was not breathing when he or she entered the water will have undiluted blood in the heart. Bodies examined shortly after the time of death may have stiff jaws and limbs. Such stiffness, or rigor mortis, is evident about ten hours after death, but disappears after about a day when the tissues begin to decay at normal temperatures. Each case is different, of course, and a skillful coroner can often discover evidence that the killer never suspected he or she had left behind.

Modern crime labs are equipped with various expensive analytical devices usually associated with research conducted by chemists and physicists. Scanning electron microscopes are used to magnify surfaces by as much as a factor of 200,000. Because the material being scanned emits x rays as well as secondary electrons in response to the electrons used in the scanning process, the microscope can be used together with an x ray micro analyzer to identify elements in the surface being scanned. The technique has been particularly successful in detecting the presence of residues left when a gun is fired.

The mass spectrometer and the gas chromatograph have been particularly effective in separating the components in illegal drugs, identifying them, and providing the data needed to track down their source and origin. Thin layer chromatography (TLC) has proved useful in identifying colored fibers. Although many fibers may appear identical under the microscope, they can often be distinguished by separating the component dyes used in coloring the fabric. Fusion microscopy—using changes in birefringence with temperature—has also proved useful in identifying and comparing synthetic fibers found at crime scenes. In addition to using such physical properties as density, dispersion, and refractive index to match and identify glass samples, the plasma emission spectroscope has proven helpful in analyzing the component elements in glass as well as distinguishing among various types of glass found in windows, bottles, and windshields.

The role and impact of forensic sciences came to the forefront of public attention during the highly publicized and televised O.J. Simpson murder trial. Lawyers for both sides offered a wide variety of forensic evidence—and disputed the validity of opposing forensic evidence—before the controversial acquittal of Simpson.

In 2002, forensic science specialists played an integral role in the tracking and eventual identification of evidence (e.g. similarities in ballistics , psychological profiles, etc.) that allowed investigators link a nationwide a string of crimes that culminated in several snipers attacks in the Washington-Virginia-Maryland area.

A new and emerging area of forensic science involves the reconstruction of computer data. High speed and large memory capacity computers also allow for what forensic investigators term as "virtual criminality," the ability of computer animation to recreate crime scenes.

Resources

books

Butler, John M. Forensic DNA Typing: The Biology and Technology Behind STR Markers Academic Press, 2001.

Lee, Henry C., and Charles D. Gill. Cracking Cases: The Science of Solving Crimes. Prometheus Books, 2002.

Nordby, Jon J. Dead Reckoning: The Art of Forensic Detection. CRC Press, 2000.

Sachs, Jessica S. Corpse: Nature, Forensics, and the Struggle toPinpoint Time of Death. An Exploration of the Haunting Science of Forensic Ecology. Perseus Publishing, 2001.

Saferstein, Richard. Criminalistics: An Introduction to ForensicScience. New York: Prentice-Hall, 2000.

other

Consulting and Ducation in Forensic Science. "Forensic Science Timeline." Norah Rudin [cited March 16, 2003]. <http://www.forensicdna.com/Timeline.htm.>.

Robert Gardner

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Birefringence: —Splitting of light into two separate beams in which the light travels at different speeds.
Gas chromatograph: —A device that separates and analyzes a mixture of compounds in gaseous form.
Mass spectrometer: —A device that uses a magnetic field to separate ions according to their mass and charge.
Polymorphic: —Distinct inherited traits that are different among members of the same species. Blood groups, for example, are polymorphic, but weight is not.
Scanning electron microscope: —A device that emits a focused beam of electrons to scan the surface of a sample. Secondary electrons released from the sample are used to produce a signal on a cathode ray tube where the enlarged image is viewed.

The Gale Encyclopedia of Science

Forensic Science

views updated Jun 08 2018

Forensic Science

█ AGNIESZKA LICHANSKA

Forensic science is a multidisciplinary subject used for examining crime scenes and gathering evidence to be used in prosecution of offenders in a court of law. Forensic science techniques are also used to examine compliance with international agreements regarding weapons of mass destruction.

The main areas used in forensic science are biology, chemistry, and medicine, although the science also includes the use of physics, computer science, geology, and psychology. Forensic scientists examine objects, substances (including blood or drug samples), chemicals (paints, explosives, toxins), tissue traces (hair, skin), or impressions (fingerprints or tidemarks) left at the crime scene. The majority of forensic scientists specialize in one area of science.

Evidence and Trace Examination

The analysis of the scene of crime or accident involves obtaining a permanent record of the scene (forensic photography) and collection of evidence for further examination and comparison. Collected samples include biological (tissue samples such as skin, blood, semen, or hair), physical (fingerprints, shells, fragments of instruments or equipment, fibers, recorded voice messages, or computer discs) and chemical (samples of paint, cosmetics, solvents, or soil).

Most commonly, the evidence collected at the scene is subsequently processed in a forensic laboratory by scientists specializing in a particular area. Scientists identify, for example, fingerprints, chemical residues, fibers, hair, or DNA left behind. However, miniaturization of equipment and the ability to perform most forensic analysis at the scene of crime results in more specialists being present in the field. Presence of more people at the scene of crime introduces a greater likelihood of introduction of contamination into the evidence. Moreover, multi-handling of a piece of evidence (for example a murder weapon being analyzed by many specialists) is also likely to introduce traces of tissue or DNA not originating from the scene of a crime. All this results in strict quality controls imposed on collection, handling, and analysis of evidence to ensure lack of contamination. For example, in DNA analysis it is essential that samples are stored at the correct temperature and that there is no contamination from a person handling a sample by wearing clean gloves and performing analysis in a clean laboratory.

Ability to properly collect and process forensic samples can affect the ability of the prosecution to prove their case during a trial. The presence of chemical traces or DNA on a piece of debris is also crucial in establishing the chain of events leading to a crime or accident.

A growing area of forensic analysis is monitoring non-proliferation of weapons of mass destruction, analysis of possible terrorist attacks or breaches of security. The nature of samples analyzed is wide, but slightly different to a criminal investigation. In addition to the already described samples, forensic scientists who gather evidence of mass destruction collect swabs from objects, water, and plant material to test for the presence of radioactive isotopes, toxins, or poisons, as well as chemicals that can be used in production of chemical weapons. The main difference from the more common forensic investigation is the amount of chemicals present in a sample. Samples taken from the scene of suspected chemical or biological weapons often contain minute amounts of chemicals and require very sensitive and accurate instruments for analysis.

Biological traces. Biological traces are collected not only from the scene of crime and a deceased person, but also from surviving victims and suspects. Most common samples obtained are blood, hair, and semen. DNA can be extracted from any of these samples and used for comparative analysis.

DNA is the main method of identifying people. Victims of crashes or fires are often unrecognizable, but adequate DNA can be isolated and a person can be positively identified if a sample of their DNA or their family's

DNA is taken for comparison. Such methods are being used in the identification of the remains in Yugoslav war victims, the World Trade Center terrorist attack victims, and the 2002 Bali bombing victims.

Biological traces, investigated by forensic scientists come from bloodstains, saliva samples (from cigarette buts or chewing gum) and tissue samples, such as skin, nails, or hair. Samples are processed to isolate the DNA and establish the origin of the samples. Samples must first be identified as human, animal, or plant before further investigation proceeds. For some applications, such as customs and quarantine, traces of animal and plant tissue have to be identified to the level of the species, as transport of some species is prohibited. A presence of a particular species can also prove that a suspect or victim visited a particular area. In cases of national security, samples are tested for the presence of pathogens and toxins, and the latter are also analyzed chemically.

Chemical traces. Forensic chemistry performs qualitative and quantitative analysis of chemicals found on people, various objects, or in solutions. The chemical analysis is the most varied from all the forensic disciplines. Chemists analyze drugs as well as paints, remnants of explosives, fire debris, gun shot residues, fibers, and soil samples. They can also test for a presence of radioactive substances (nuclear weapons), toxic chemicals (chemical weapons) and biological toxins (biological weapons). Forensic chemists can also be called on in a case of environmental pollution to test the compounds and trace their origin. The samples are obtained from a variety of objects and often contain only minute amounts of chemicals.

The identification of fire accelerants such as kerosene or gasoline is of great importance for determining the cause of a fire. Debris collected from a fire must be packed in tight, secure containers, as the compounds to be analyzed are often volatile. An improper transport of such debris would result in no detection of important traces. One of the methods used for this analysis involves the use of charcoal strips. The chemicals from the debris are absorbed onto the strip and subsequently dissolved in a solvent before analysis. This analysis allows scientists to determine the hydrocarbon content of the samples and identify the type of fire accelerator used.

Physical evidence. Physical evidence usually involves objects found at the scene of a crime. Physical evidence may include all sorts of prints such as fingerprints, footprints, handprints, tidemarks, cut marks, tool marks, etc. Analysis of some physical evidence is conducted by making impressions in plaster, taking images of marks, or lifting the fingerprints from objects encountered. These serve later as a comparison to identify, for example, a vehicle that was parked at the scene, a person that was present, a type of manufacturing method used to create a tool, or a method used to break in a building or harm a victim.

An examination of documents found at the scene or related to the crime is often an integral part of forensic analysis. Such examination is often able to establish not only the author, but more importantly identify any alterations that have taken place. Specialists are also able to recover text from documents damaged by accident or on purpose.

Identification. The identification of people can be performed by fingerprint analysis or DNA analysis. When none of these methods can be used, the facial reconstruction can be used instead to generate a person's image. TV and newspapers then circulate the image for identification.

Other Fornsic Scientists

Pathologists and forensic anthropologists play a very important part in forensic examination. They are able to determine the cause of death by examining marks on the bone(s), skin (gunshot wounds), and other body surfaces for external trauma. They can also determine a cause of death by toxicological analysis of blood and tissues.

A number of analytical methods are used by forensic laboratories to analyze evidence from a crime scene. Methods vary, depending on the type of evidence analyzed and information that needs to be extracted from the traces found. If a type of evidence is encountered for the first time, a new method is developed.

Biological samples are most commonly analyzed by polymerase chain reaction (PCR). The results of PCR are then visualized by gel electrophoresis. Forensic scientists tracing the source of a biological attack could use the new hybridization or PCR-based methods of DNA analysis. Biological and chemical analysis of samples can identify toxins found.

Imaging used by forensic scientists can be as simple as a light microscope, or can involve an electron microscope, absorption in ultraviolet to visible range, color analysis or fluorescence analysis. Image analysis is used not only in cases of biological samples, but also for analysis of paints, fibers, hair, gunshot residue, or other chemicals. Image analysis is often essential for an interpretation of physical evidence. Specialists often enhance photographs to visualize small details essential in forensic analysis. Image analysis is also used to identify details from surveillance cameras.

The examination of chemical traces often requires very sensitive chromatographic techniques or mass spectrometric analysis. Four major types of chromatographic methods used are: thin layer chromatography (TLC) to separate inks and other chemicals, atomic absorption chromatography for analysis of heavy metals, gas chromatography (GC), and liquid chromatography (HPLC). GC is most widely used in identification of explosives, accelerators, propellants, and drugs or chemicals involved in chemical weapon production, while liquid chromatography (HPLC) is used for detection of minute amounts of compounds in complex mixtures. These methods rely on separation of the molecules based on their ability to travel in a solvent (TLC) or to adhere to adsorbent filling the chromatography column. The least strongly absorbed compounds are eluted first and the most tightly bound last. By collecting all of the fractions and comparing the observed pattern to standards, scientists are able to identify the composition of even the most complex mixtures.

New laboratory instruments are able to identify nearly every element present in a sample. Because the composition of alloys used in production of steel instruments, wires or bullet casings is different between various producers, it is possible to identify a source of the product.

In some cases chromatography alone is not an adequate method for identification. It is then combined with another method to separate the compounds even further and results in greater sensitivity. One such method is mass spectrometry (MS). A mass spectrometer uses high voltage to produce charged ions. Gaseous ions or isotopes are then separated in a magnetic field according to their masses. A combined GC-MS instrument has a very high sensitivity and can analyze samples present at concentrations of one part-per-billion.

As some samples are difficult to analyze with MS alone, a laser vaporization method (imaging laser-ablation mass spectroscopy) was developed to produce small amounts of chemicals from solid materials (fabrics, hair, fibers, soil, glass) for MS analysis. Such analysis can examine hair samples for presence of drugs or chemicals. Due to its high sensitivity, the method is of particular use in monitoring areas and people suspected of production of chemical, biological or nuclear weapons, or narcotics producers.

While charcoal sticks are still in use for fire investigations, a new technology of solid-phase microextraction (SPME) was developed to collect even more chemicals and does not require any solvent for further analysis. The method relies on the use of sticks similar to charcoal, but coated with various polymers for collecting different chemicals (chemical warfare agents, explosives, or drugs). Collected samples are analyzed immediately in the field in by GC.

A number of instruments used are smaller than ever before, allowing them to be used directly in the field with rapid results. For example, a combined GC-MS analysis device can analyze a sample within 15 minutes directly in the field. The standard laboratory instrument is large with a weight over 100 kilograms, while the portable version is only 28 kilograms. A number of government agencies (for example the FBI) are now armed with the portable instruments and can perform rapid forensic analysis in the field in a time shorter than it would take to transport samples to a forensic laboratory. United States troops are equipped with similar instruments on board some tanks and trucks, in order to quickly determine the presence of chemical or biological weapons on the battlefield

Applications of forensic science. The main use of forensic science is for purposes of law enforcement to investigate crimes such as murder, theft, or fraud. Forensic scientists are also involved in investigating accidents such as train or plane crashes to establish if they were accidental or a result of foul play. The techniques developed by forensic science are also used by the army to analyze the possibility of the presence of chemical weapons, high explosives or to test for propellant stabilizers. Gasoline products often evaporate rapidly and their presence cannot be confirmed, but residues of chemicals, such as propellant stabilizers, are present for much longer indicating that an engine or missile was used.

█ FURTHER READING:

Houde, John. Crime Lab: A guide for Nonscientists. Rolling Bay: Calico Press, 1998.

Kelly, John F., and Phillip K, Wearne. Tainting Evidence: Inside the Scandals at the FBI Crime Lab. New York: Free Press, 1998.

Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. New York: Prentice-Hall, 2000.

ELECTRONIC:

American Academy of Forensic Science <http://www.aafs.org.> (7 February 2003).

Consulting and Ducation in Forensic Science. "Forensic Science Timeline." Norah Rudin. <http://www.forensicdna.com/Timeline.htm.>(7 February 2003).

Forensic Science Center, University of California Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550–9234. (925) 423–1189. <http://www.llnl.gov/IPandC/op96/10/10h-for.html.> (7 February 2003).

Forensic Science Web Pages. 7 February 1997. <http://home.earthlink.net/~thekeither/Forensic/forsone.htm.>(7 February 2003).

National Center for Forensic Science, University of Central Florida 12354 Research Parkway Orlando, FL 32826.(407) 823–6469. <http://ncfs.ucf.edu/navbar.html.> (7 February 2003).

Forensic Science

views updated May 29 2018

Forensic Science

Forensic science is a term used to describe the actions taken by investigators in multidisciplinary fields for the examination of crime scenes and gathering of evidence to be used in prosecution of offenders in a court of law. The main use of forensic science is for purposes of law enforcement to investigate crimes such as murder , theft, or fraud. Forensic scientists are also involved in investigating accidents such as train or plane crashes to establish if they were accidental or a result of foul play. The techniques developed by forensic science are also used by the U.S. military to analyze the possibility of the presence of chemical weapons or high explosives , to test for propellant stabilizers, or to monitor compliance with international agreements regarding weapons of mass destruction.

The main areas used in forensic science are biology, chemistry, and medicine , although the science also includes the use of physics, computer science, geology , or psychology . Forensic scientists examine objects, substances (including blood or drug samples), chemicals (paints, explosives, toxins ), tissue traces (hair, skin), or impressions (fingerprints or tidemarks) left at the crime scene. The majority of forensic scientists specialize in one area of science.

The analysis of the scene of a crime or accident involves obtaining a permanent record of the scene (forensic photography ) and collection of evidence for further examination and comparison. Collected samples include biological (tissue samples such as skin, blood, semen , or hair), physical (fingerprints, shells, fragments of instruments or equipment, fibers , recorded voice messages, or computer discs) and chemical (samples of paint, cosmetics, solvents, or soil).

Most commonly, the evidence collected at the scene is subsequently processed in a forensic laboratory by scientists specializing in a particular area. Scientists identify, for example, fingerprints, chemical residues, fibers, hair, or DNA . However, miniaturization of equipment and the ability to perform most forensic analysis at the scene of crime results in more specialists being present in the field. Presence of more people at the scene of crime introduces a greater likelihood of introduction of contamination into the evidence. Moreover, multi-handling of a piece of evidence (for example, a murder weapon) is also likely to introduce traces of tissue or DNA not originating from the scene of a crime. Consequently, strict quality controls are imposed on collection, handling, and analysis of evidence to avoid contamination.

The ability to properly collect and process forensic samples can affect the ability of the prosecution to prove their case during a trial. The presence of chemical traces or DNA on a piece of debris is also crucial in establishing the chain of events leading to a crime or accident.

Biological traces are collected not only from the crime scene and deceased person, but also from surviving victims and suspects. Most commonly, samples obtained are blood, hair, and semen. DNA can be extracted from any of these samples and used for comparative analysis.

DNA is the main method of identifying people. Victims of crashes or fires are often unrecognizable, but if adequate DNA can be isolated a person can be positively identified if a sample of their DNA or their family's DNA is taken for comparison. Such methods are being used in the identification of the remains in Yugoslav war victims, the World Trade Center terrorist attack victims, and the 2002 Bali bombing victims.

Biological traces, investigated by forensic scientists come from bloodstains, saliva samples (from cigarette butts or chewing gum) and tissue samples, such as skin, nails, or hair. Samples are processed to isolate the DNA and establish the origin of the samples. Samples must first be identified as human, animal, or plant before further investigation proceeds. For some applications, such as customs and quarantine, traces of animal and plant tissue have to be identified to the level of the species, as transport of some species is prohibited. A presence of a particular species can also prove that a suspect or victim visited a particular area. In cases of national security, samples are tested for the presence of pathogens and toxins, and the latter are also analyzed chemically.

A growing area of forensic analysis is monitoring non-proliferation of weapons of mass destruction, analysis of possible terrorist attacks, or breaches of security. The nature of samples analyzed is wide, but slightly different from a criminal investigation. In addition to the already-described samples, forensic scientists who gather evidence of weapons of mass destruction collect swabs from objects, water, and plant material to test for the presence of radioactive isotopes, toxins, or poisons, as well as chemicals that can be used in production of chemical weapons. The main difference from the more common forensic investigation is the amount of chemicals present in a sample. Samples taken from the scene of suspected chemical or biological weapons often contain minute amounts of chemicals and require very sensitive and accurate instruments for analysis.

Forensic chemistry performs qualitative and quantitative analysis of chemicals found on people, various objects, or in solutions. The chemical analysis is the most varied from all the forensic disciplines. Chemists analyze drugs as well as paints, remnants of explosives, fire debris , gunshot residues, fibers, and soil samples. They can also test for a presence of radioactive substances (nuclear weapons), toxic chemicals (chemical weapons), and biological toxins (biological weapons). Forensic chemists can also be called on in a case of environmental pollution to test the compounds and trace their origin.

The identification of fire accelerants such as kerosene or gasoline is of great importance for determining the cause of a fire. Debris collected from a fire must be packed in tight, secure containers, as the compounds to be analyzed are often volatile. An improper transport of such debris would result in no detection of important traces. One of the methods used for this analysis involves the use of charcoal strips. The chemicals from the debris are absorbed onto the strip and subsequently dissolved in a solvent before analysis. This analysis allows scientists to determine the hydrocarbon content of the samples and identify the type of fire accelerator used.

Physical evidence usually refers to objects found at the scene of a crime. Physical evidence may include all sorts of prints such as fingerprints, footprints, handprints, tidemarks, cut marks, tool marks, etc. Analysis of some physical evidence is conducted by making impressions in plaster, taking images of marks, or lifting the fingerprints from objects encountered. These serve later as a comparison to identify, for example, a vehicle that was parked at the scene, a person that was present, a type of manufacturing method used to create a tool, or a method used to break in a building or harm a victim.

An examination of documents found at the scene or related to the crime is often an integral part of forensic analysis. Such examination is often able to establish not only the author but, more importantly, identify any alterations that have taken place. Specialists are also able to recover text from documents damaged by accident or on purpose.

The identification of people can be performed by fingerprint analysis or DNA analysis. When none of these methods is viable, facial reconstruction can be used instead to generate a person's image. Television and newspapers then circulate the image for identification.

Pathologists and forensic anthropologists play a very important part in forensic examination. They are able to determine the cause of death by examining marks on the bone(s), skin (gunshot wounds), and other body surfaces for external trauma. They can also determine a cause of death by toxicological analysis of blood and tissues.

A number of analytical methods are used by forensic laboratories to analyze evidence from a crime scene. Methods vary, depending on the type of evidence analyzed and information extracted from the traces found. If a type of evidence is encountered for the first time, a new method is developed.

Biological samples are most commonly analyzed by polymerase chain reaction (PCR ). The results of PCR are then visualized by gel electrophoresis . Forensic scientists tracing the source of a biological attack could use the new hybridization or PCR-based methods of DNA analysis. Biological and chemical analysis of samples can identify toxins found.

Imaging used by forensic scientists can be as simple as a light microscope, or can involve an electron microscope, absorption in ultraviolet to visible range, color analysis, or fluorescence analysis. Image analysis is used not only in cases of biological samples, but also for analysis of paints, fibers, hair, gunshot residue , or other chemicals. Image analysis is often essential for an interpretation of physical evidence. Specialists often enhance photographs to visualize small details essential in forensic analysis. Image analysis is also used to identify details from surveillance cameras .

The examination of chemical traces often requires very sensitive chromatographic techniques or mass spectrometric analysis. The four major types of chromatographic methods used are: thin layer chromatography (TLC) to separate inks and other chemicals; atomic absorption chromatography for analysis of heavy metals; gas chromatography (GC); and liquid chromatography (HPLC). GC is most widely used in identification of explosives, accelerators, propellants, and drugs or chemicals involved in chemical weapon production, while liquid chromatography (HPLC) is used for detection of minute amounts of compounds in complex mixtures. These methods rely on separation of the molecules based on their ability to travel in a solvent (TLC) or to adhere to adsorbent filling the chromatography column. By collecting all of the fractions and comparing the observed pattern to standards, scientists are able to identify the composition of even the most complex mixtures.

New laboratory instruments are able to identify nearly every element present in a sample. Because the composition of alloys used in production of steel instruments, wires, or bullet casings is different between various producers, it is possible to identify a source of the product.

In some cases chromatography alone is not an adequate method for identification. It is then combined with another method to separate the compounds even further and results in greater sensitivity. One such method is mass spectrometry (MS). A mass spectrometer uses high voltage to produce charged ions. Gaseous ions or isotopes are then separated in a magnetic field according to their masses. A combined GC-MS instrument has a very high sensitivity and can analyze samples present at concentrations of one part-per-billion.

As some samples are difficult to analyze with MS alone, a laser vaporization method (imaging laser-ablation mass spectroscopy ) was developed to produce small amounts of chemicals from solid materials (fabrics, hair, fibers, soil, glass ) for MS analysis. Such analysis can examine hair samples for presence of drugs or chemicals. Due to its high sensitivity, the method is of particular use in monitoring areas and people suspected of production of chemical, biological, or nuclear weapons, or narcotics producers.

While charcoal sticks are still in use for fire investigations, a new technology of solid-phase microextraction (SPME) was developed to collect even more chemicals and does not require any solvent for further analysis. The method relies on the use of sticks similar to charcoal, but coated with various polymers for collecting different chemicals (chemical warfare agents, explosives, or drugs). Collected samples are analyzed immediately in the field by GC.

see also Computer forensics; Crime scene investigation; DNA; DNA recognition instruments; Document forgery; Gas chromatograph-mass spectrometer; Isotopic analysis; Thin layer chromatography.

World of Forensic Science

Forensic Science

views updated May 29 2018

FORENSIC SCIENCE

The application of scientific knowledge and methodology to legal problems and criminal investigations.

Sometimes called simply forensics, forensic science encompasses many different fields of science, including anthropology, biology, chemistry, engineering, genetics, medicine, pathology, phonetics, psychiatry, and toxicology.

The related term criminalistics refers more specifically to the scientific collection and analysis of physical evidence in criminal cases. This includes the analysis of many kinds of materials, including blood, fibers, bullets, and fingerprints. Many law enforcement agencies operate crime labs that perform scientific studies of evidence. The largest of these labs is run by the federal bureau of investigation.

Forensic scientists often present expert testimony to courts, as in the case of pathologists who testify on causes of death and engineers who testify on causes of damage from equipment failure, fires, or explosions.

Modern forensic science originated in the late nineteenth century, when European criminal investigators began to use fingerprinting and other identification techniques to solve crimes. As the field of science expanded in scope throughout the twentieth century, its application to legal issues became more and more common. Because nearly every area of science has a potential bearing on the law, the list of areas within forensic science is long.

Forensic Medicine and Psychology

Forensic medicine is one of the largest and most important areas of forensic science. Also called legal medicine or medical jurisprudence, it applies medical knowledge to criminal and civil law. Areas of medicine that are commonly involved in forensic medicine are anatomy, pathology, and psychiatry.

Many law enforcement agencies employ a forensic pathologist, sometimes called a medical examiner, who determines the causes of sudden or unexpected death. Forensic toxicologists, who study the presence of poisons or drugs in the deceased, often help forensic pathologists. Forensic odontologists, or dentists, analyze dental evidence to identify human remains and the origin of bite marks.

Forensic medicine is often used in civil cases. The cause of death or injury is considered in settling insurance claims or medical malpractice suits, and blood tests often contribute to a court's decision in cases attempting to determine the paternity of a child.

Forensic Science in the Federal Bureau of Investigation

Since its establishment in 1932, the FBI Laboratory has been a world leader in the scientific analysis of physical evidence related to crime. From its location in the J. Edgar Hoover FBI Building, in Washington, D.C., the laboratory provides a wide range of free forensic services to U.S. law enforcement agencies. The laboratory is divided into several major departments: the Document Section, Scientific Analysis Section, Special Projects Section, Latent Fingerprint Section, and Forensic Science Research and Training Center.

The laboratory's Document Section examines paper documents, ink, shoe and tire tread designs, and other forms of evidence related to a wide variety of crimes, including forgery and money laundering. It performs linguistic analysis of documents to determine authorship. It also evaluates the validity and danger of written threats. Its Computer Analysis and Response Team recovers evidence, including encrypted information, from computer systems—evidence that is crucial to the prosecution of white-collar crime. The Document Section also maintains files of bank robbery notes, anonymous extortion letters, and office equipment specifications.

The Scientific Analysis Section has seven divisions: Chemistry Toxicology, DNA Analysis/Serology, Elemental and Metals Analysis, Explosives, Firearms-Toolmarks, Hairs and Fibers, and Materials Analysis. This section's analysis of blood, semen, and saliva assists the investigation of violent crimes such as murder, rape, assault, and hit-and-run driving. Its research also provides insight into many other crimes, including bombings, arson, drug tampering, and poisoning.

The services provided by the Special Projects Section include composite sketches of suspects, crime scene drawings and maps, videotape and audiotape analysis and enhancement, and analysis of electronic devices such as wiretaps and listening devices.

The Latent Fingerprint Section examines evidence for hidden fingerprints, palm prints, footprints, and lip prints.

The Forensic Science Research and Training Center offers classes to law enforcement officials from the United States and other countries. These classes cover DNA analysis, the detection and recovery of human remains, arson and bomb blast investigation, and many other topics.

To better perform its research, the laboratory maintains files on many kinds of physical evidence, including adhesives, ammunition, paint, and office equipment. The laboratory also provides experts who will furnish testimony on the nature of the evidence.

The laboratory publishes the Handbook of Forensic Science to explain its forensic services to law enforcement agencies. The handbook outlines procedures for safely and effectively gathering evidence from crime scenes and shipping it to the laboratory for analysis.

Mental health and psychology professionals have contributed a great deal to the legal understanding of issues such as the reliability of eyewitness testimony, responsibility for criminal behavior, and the process of decision making in juries. These professionals include those with a medical degree, such as psychiatrists, neurologists, and neuropsychologists, as well as individuals without a medical degree, such as psychologists.

Mental health professionals are frequently consulted in civil and criminal cases to help determine an individual's state of mind with regard to a crime, the validity of testimony before a court, or an individual's competence to stand trial or make a legal decision. Their input may also be vital to legal procedures for deciding whether to commit a person to an institution because of mental illness, or to allow a person to leave an institution for those who are mentally ill.

Forensic neuropsychology is a specialized area of forensic medicine that applies the functioning of the nervous system and brain to legal issues involving mind and behavior. Equipped with an improved understanding of how the brain works and influences behavior, neuropsychologists have increasingly been asked to provide testimony to courts attempting to determine whether a criminal act is a result of a nervous system dysfunction. They also testify as to the reliability of witness testimony given by victims of crime, the competency of individuals to stand trial, the likelihood that a condition of mental retardation or brain injury predisposed an individual to commit a crime, the possibility that an individual has verifiable memory loss, and various aspects of dementias and other brain disorders caused by AIDS, head injuries, and drugs, alcohol, and other chemicals.

In civil cases, the work of neuropsychologists has been used to determine whether a defendant's wrongdoing caused a plaintiff's injury. In family courts, neuropsychologists assess brain damage in children who have been physically abused.

Forensic psychologists provide expert testimony that touches on many of the same areas as that given by forensic psychiatrists and neuropsychologists. In addition, psychologists consult with the legal system on issues such as correctional procedures and crime prevention. In 1962, a U.S. court of appeals issued an influential decision that established the ability of a psychologist to testify as an expert witness in a federal court of law (Jenkins v. United States, 113 U.S. App. D.C. 300, 307 F.2d 637 [D.C. Cir. 1962]). Before that time, expert testimony on mental health was largely restricted to physicians.

Other Areas of Forensic Science

Forensic engineers provide courts with expertise in areas such as the design and construction of buildings, vehicles, electronics, and other items. Forensic linguists determine the authorship of written documents through analyses of handwriting, syntax, word usage, and grammar. Forensic anthropologists identify and date human remains such as bones. Forensic geneticists analyze human genetic material, or DNA, to provide evidence that is often used by juries to determine the guilt or innocence of criminal suspects. Forensic phoneticians deal with issues such as the validity of tape-recorded messages, the identification of speakers on recorded messages, the enhancement of recorded messages, the use of voiceprints, and other aspects of electronic surveillance.

cross-references

DNA Evidence.

West's Encyclopedia of American Law

Forensic Science

views updated May 29 2018

FORENSIC SCIENCE

The word forensic is derived from the Latin word forensic—a reference to Roman court forums in which evidence of wrongdoing was presented. Modern use of the term forensics refers to scientific principles and processes that are applied in the analysis of evidence for legal purposes. Alternatively known as criminalistics, forensics involves using sophisticated techniques and tools to identify, collect, analyze, preserve, and present evidence of crimes or civil wrongdoing in legal proceedings, as well as to verify identification of deceased individuals. The essential goal of forensics analysis is to verify connections between two or more physical items, for example, the blood of a homicide victim to that found on clothes worn by a suspect. Forensics involves analysis of many other types of evidentiary items such as prescription and illicit/illegal drugs, metals, glass, plastics, fuels, paints, tire/shoe prints, tool/tool marks, and latent substances such as synthetic fibers, human hair, and animal fur, among others.

Modern forensics began with nineteenth-century efforts of Alphonse Bertillon (1853–1914), director of the Bureau of Criminal Identification of the Paris (France) Police Department, to classify and identify criminals on the basis of their physical characteristics. In 1888 Francis Galton proposed a fingerprint classification method after which fingerprinting was first used for criminal identification by Scotland Yard investigators in 1901, and by New York City detectives in 1902. By 1930 the Federal Bureau of Investigation (FBI) of the U.S. Department of Justice had established a national fingerprint classification system, and in 1946 the FBI created its Identification Division that relied extensively on burgeoning fingerprint records for suspect identification in criminal cases. Since then the FBI lab has helped solve thousands of criminal cases using many forensics analysis methods, and is among the largest and most technologically capable forensic laboratories in the world.

Types of Forensics Evidence and Analysis

There are many types of forensic methods, each of which corresponds to the kind of evidence analyzed. For example, ballistics is the study of firearms, ammunition, bombs/explosives, bullets, and other projectiles. Forensic anthropology attempts to reconstruct the likeness of decomposed or dismembered bodies based on skeletal remains and other factors. Forensic odontology matches bite marks with teeth or dental records; and forensic entomologists study corpses infested with insects to determine the approximate time of death and other information. Forensic psychology and psychiatry seek to profile criminals, and also apply social work and mental health counseling practices to investigative situations. Forensic toxicology involves analysis of intoxicants, drugs, and poisons. Forensic taphonomy pertains to the examination of dead and decaying human, animal, and plant remains.

The most modern, prominent, and scientifically promising form of forensics is DNA analysis profiling which involves comparison of deoxyribonucleic acid found in human body tissue or fluids such as blood, perspiration, urine, semen, or vaginal secretions. In addition, biometrics analysis is used in forensics to verify identification of people by comparing biological traits such as finger/palm prints and iris or retina cell patterns. Other forms of forensics involve toxicology (the study of poisons and their harmful effects), computer forensics, voiceprint identification, and polygraph examinations (lie detector testing). In addition to determining the sources of criminal evidence and matching these to known sources, forensics also involves crime scene reconstruction—examining evidence to determine the nature of activities and physical dynamics of interactions among perpetrators and crime victims, series of events, directions of travel, angles and relative forces of impact, pre/post impact trajectories, and primary versus secondary causes of harm, and more.

Fundamental and Ethical Challenges in Forensics

Primary challenges in forensics pertaining to the overall validity, reliability, and credibility of evidence presented in court cases involves:

protecting evidence from harm before, during, and after its collection at crime scenes and in laboratories and evidence storage facilities;
accurately analyzing evidence and truthfully presenting findings in legal proceedings to help explain how crimes occurred and the possible guilt or innocence of individuals accused of crimes;
developing and maintaining expertise of forensics professionals through training;
acquiring, certifying, and maintaining laboratory equipment;
providing managerial oversight to ensure accurate analyses and truthful reporting of findings in legal proceedings;
truthfully testifying about analytical methods, findings, and credentials of examiners;
achieving laboratory accreditation by one or more nationally recognized professional membership associations.

Criticism of and concern about forensics analysis has involved all the challenges listed above. In addition, so-called voodoo science or junk science refers to the reality that all forms of forensics analysis require professional judgment in determining whether evidence collected at crime scenes matches known-source samples to the exclusion of all other possibilities. In many types of forensics analysis there is no scientific basis for employing statistical probability modeling to accurately estimate the chances that one or more evidentiary items are not aperfect match. Fingerprint analysis, for example, although long accepted by courts as a type of scientific evidence is actually a technical art predicated on the belief that no two people have exactly the same print patterns and that professionals conducting tests sought exculpatory evidence in addition to match points. This fundamental problem extends to other types of forensics analysis, and when combined with numerous legal cases in which forensics experts lied about their analytical findings and/or professional credentials, has resulted in considerable controversy about the reliability of evidence collection and forensics analysis procedures, and the trustworthiness of testimony in legal proceedings about forensic analysis/laboratory findings.

In Daubert v. Merrell Dow Pharmaceuticals, Inc. (1993), the U.S. Supreme Court scrutinized the field of forensics and established new legal standards regarding the admissibility of scientific evidence and expert witness testimony provided by forensics professionals. Standardized DNA evidence gathering and analysis championed by the National Institute of Justice of the U.S. Department of Justice, and acceptance of this form of truly scientific evidence by federal, state, and local level criminal justice systems, is important to the future of forensics, as are quality control standards such as those established by the American Society of Crime Lab Directors/Laboratory Accreditation Board. Ultimately the usefulness and reliability of forensics evidence in legal proceedings will depend on ethical (and potentially government regulated) use of forensics technologies in the public sector and in privately owned or operated laboratories.

SAMUEL C. MCQUADE, III

BIBLIOGRAPHY

Bevel, Tom. (1997). Bloodstain Pattern Analysis: With an Introduction to Crime Scene Reconstruction. Boca Raton, FL: CRC Press.

Cole, Simon A. (2001). Suspect Identities: A History of Criminal Identification and Fingerprinting. Cambridge, MA: Harvard University Press.

Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 US 579 (1993).

Fisher, David. (1995). Hard Evidence: How Detectives Inside the FBI's Sci-Crime Lab Have Helped Solve America's Toughest Cases. New York: Simon & Schuster.

Galton, Francis. (1888). "Personal Identification and Description." Nature 201–202.

James, Stuart H., and Jon J. Nordby, eds. (2003). Forensic Science: An Introduction to Scientific and Investigative Techniques. Boca Raton: CRC Press.

National Research Council. (1992). DNA Technology on Forensic Science. Washington, DC: National Academy Press.

Schmalleger, Frank. (2005). "The Future of Criminal Justice." In Criminal Justice Today: An Introductory Text For The Twenty-First Century, 8th edition. Upper Saddle River, NJ: Pearson/Prentice Hall.

Encyclopedia of Science, Technology, and Ethics

Forensic Science

views updated Jun 11 2018

Forensic science

Forensic science is the application of science to matters of law. A basic principle of forensic science is that a criminal always brings something to the scene of a crime that he or she leaves behind. The "something" left behind is the evidence that detectives and criminalists (people who make use of science to solve crimes) look for. It might be fingerprints, footprints, teeth marks, blood, semen, hair, fibers, broken glass, a knife or gun, or a bullet. It also might be something less tangible such as the nature of the wounds or bruises left on a victim's body, which might indicate the nature of the weapon or the method of assault. Careful analysis of evidence left at the scene of a crime often can be used in establishing the guilt or innocence of someone on trial.

Fingerprints

Although fingerprints have been used by crime investigators for more than a century, they remain one of the most sought after pieces of evidence. All human beings are born with a characteristic set of ridges on their fingertips. The ridges, which are rich in sweat pores, form a pattern that remains fixed for life. Even if the skin is removed, the same pattern will be evident when the skin regenerates. Some of the typical patterns found in fingerprints are arches, loops, and whorls.

Oils from sweat glands collect on these ridges. When we touch something, a small amount of the oils and other materials on our fingers are left on the surface of the object we touched. The pattern left by these substances, which collect along the ridges on our fingers, make up the fingerprints that police look for at the scene of a crime. Fingerprints collected as evidence can be compared with fingerprints on file or taken from a suspect. The Federal Bureau of Investigation (FBI) maintains a fingerprint library with patterns taken from more than 10 percent of the entire United States population.

Fingerprints are not the only incriminating patterns that a criminal may leave behind. Lip prints are frequently found on glasses. Footprints and the soil left on the print may match those found in a search of an accused person's premises. Tire tracks, bite marks, toe prints, and prints left by bare feet also may provide useful evidence. In cases where identifying a victim is difficult because of tissue decomposition or death caused by explosions or extremely forceful collisions, a victim's teeth may be used for comparison with dental records.

Genetic fingerprinting

The nuclei within our cells contain coiled, threadlike bodies called chromosomes. Chromosomes are made of deoxyribonucleic acid (DNA). DNA carries the "blueprint" (genes) that directs the growth, maintenance, and activities that go on within our bodies.

Although certain large strands of DNA are the same in all of us, no two people have exactly the same DNA (except for identical twins). It is these unique strands of DNA that are used by forensic scientists to establish a characteristic pattern—the so-called genetic fingerprint. Because different people have different DNA, the prints obtained from different people will vary considerably; however, two samples from the same person will be identical. If there is a match between DNA extracted from semen found on the body of a rape victim and the DNA obtained from a rape suspect's blood, the match is very convincing evidence and may well lead to a conviction or possibly a confession.

Other evidence used in forensic science

Long before DNA was recognized as the "ink" in the blueprints of life, blood samples were collected and analyzed in crime labs. The evidence available through blood typing is not as convincing as genetic fingerprinting, but it can readily prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups: A, B, AB, or O. If a person accused of a homicide has type AB blood and it matches the type found at the crime scene, the evidence for guilt is more convincing than if a match were found for type O blood.

Bullets and the remains of tools can be used as incriminating evidence. When a bullet is fired, it moves along a spiral groove in the gun barrel. It is this groove that makes the bullet spin so that it will follow a straight path much like that of a spiraling football. The markings on a bullet made by the groove are unique to each gun. Similarly, tool marks, which are often left by burglars who pry open doors or windows, can serve as useful evidence if comparisons can be made with tools associated with a person accused of the crime. Particularly incriminating are jigsaw matches—pieces of a tool left behind that can be shown to match pieces missing from a tool in the possession of the accused.

Autopsies

An autopsy can often establish the cause and approximate time of death. Cuts, scrapes, punctures, and rope marks may help to establish the cause of death. A drowning victim will have soggy lungs, water in the stomach, and blood diluted with water in the left side of the heart. A person who was not breathing when he or she entered the water will have undiluted blood in the heart. Bodies examined shortly after the time of death may have stiff jaws and limbs. Such stiffness, or rigor mortis, is evident about ten hours after death, but disappears after about a day when the tissues begin to decay at normal temperatures. Each case is different, of course, and a skillful coroner can often discover evidence that the killer never suspected he or she had left behind.

[See also Nucleic acid ]

UXL Encyclopedia of Science

forensic science

views updated May 23 2018

forensic science (medical jurisprudence) Application of medical, scientific, or technological knowledge to the investigation of crimes. Forensic medicine involves examination of living victims and suspects, as well as the pathology of the dead. The cause of death, if there is doubt, is established at an autopsy. Forensic science developed in the early 1900s in England as a collaboration between police work and medicine. Modern developments include DNA fingerprinting.

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FORENSIC SCIENCE

Types of Forensics Evidence and Analysis

Fundamental and Ethical Challenges in Forensics

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Genetic fingerprinting

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