Fingerprint Analysis (Famous Cases)

Forensic investigators have been using fingerprint evidence as a source of identification of suspects for over a hundred years. Early work was by visual analysis of very obvious prints left at the scene of a crime. Modern forensic scientists now have a range of techniques for finding prints, cleaning up and enhancing print images, and rapidly finding a match from a database using computer technology. Fingerprint evidence is seen as one of the best types of physical evidence linking a suspect to an object or location or for establishing identity. Therefore, the forensic investigator will always search for fingerprint evidence at the scene of a crime and at related locations, such as a suspect's home or car.

A fingerprint is the pattern of ridges and related characteristics found on the fingerpads, the fleshy parts of the fingers used for touching and gripping. Each person's fingerprints are unique and stay unchanged throughout life. According to Sir Francis Galton , the nineteenth-century English anthropologist, the chances of two fingerprints being identical are as small as 64 billion to one. In over a century of forensic fingerprinting, no two prints have ever been found to be the same, even those of identical twins.

Skin is never completely dry or clean; grime, oil, and sweat on the fingerpads create fingerprints whenever a person touches something. That is why criminals, unless they are wearing gloves, leave fingerprints behind. If their hands are bloodstained, then they will leave bloody fingerprints behind, an example of a patent (visible) print. Plastic prints are fingerprint impressions made in a soft material like soap or dust. Latent fingerprints are invisible, but the forensic scientist can visualize them though special lighting or with the application of chemicals. Fingerprints have been recovered from all kinds of surfaces, even plastic bags. It would be very useful to be able to reliably detect fingerprints on human skin. So far, this been very difficult to do if more than two hours have elapsed from the time the fingerprints were made. Potential methods are being developed to recover fingerprints after longer time periods have elapsed.

A fingerprint found at the scene of a crime can be dusted with chemicals to make it easier to see and then lifted or photographed. It is then compared with the fingerprints of known offenders stored in a computer database. In the past fingerprints were classified according to the specific features that make up the unique pattern of each print. With computerized storage and retrieval systems, however, classification is not really necessary as the computer can readily scan and match the whole pattern of thousands of prints. The image of fingerprints found at the scene of a crime can readily be enhanced and clarified with scanning and digitizing technology. This means that even partial prints can be of value in identifying someone at the scene of a crime.

In 1892 Francesca Rojas, an Argentine woman, became the first person ever to be convicted on fingerprint evidence. When her two young children were found beaten to death, she tried to blame a man called Velasquez who vigorously denied the charge and, in any case, had a firm alibi. Investigator Juan Vucetich , who was intrigued by the relatively new technique of fingerprint analysis, found a bloody fingerprint on a bedroom door in Rojas' house. He sawed the portion away and then had the woman give an ink-print of her thumb. Even with only a basic understanding of fingerprint analysis, it was obvious to the investigators that the bloody print belonged to Rojas. She confessed to the crime when confronted, and admitted that she committed the murders to improve her chances of marrying her boyfriend, who was known to dislike children. Rojas was sentenced to life imprisonment.

The brutal murder in 1905 of Thomas Farrow, manager of a shop in Deptford, near London, and his wife Ann was to become a milestone case in the use of fingerprint analysis in Britain. Money had been taken and a thumbprint was found on the cash box. The Criminal Investigation Department (CID) had already built up a file of fingerprints of known criminals, but this print did not match any of them. A witness led the investigators to two brothers called Albert and Alfred Stratton. A match was found between one of the men and the print found at the scene. The court battle over the evidence was, however, lengthy. Much hung in the balance as it was the first time fingerprint evidence had been used in a murder case in Britain. After two hours of deliberation, however, the jury found the two men guilty and they were later hanged.

In 1910, Thomas Jennings was arrested on suspicion of the murder of Clarence Hiller in Chicago. The main evidence against him was fingerprints, and four experts testified at his trial. However, fingerprint evidence was still relatively new and Jennings brought an appeal questioning its admissibility. In a landmark judgment, the Illinois Supreme Court upheld the conviction, saying that fingerprints were indeed a reliable form of identification. Jennings was sentenced to death and executed on February 16, 1912. He was the first person in the United States to be convicted of murder on fingerprint evidence.

Fingerprint analysis also played a role in convicting the man responsible for an audacious theft. On August 21, 1911, Leonardo da Vinci's Mona Lisa was stolen from the Louvre Museum in Paris. There was a clear fingerprint on the glass that had protected the painting. Fingerprint pioneer Alphonse Bertillon spent many months trying to match the print to samples in his collection but to no avail. Two years after the theft, police arrested Vicenzo Perugia in connection with the crime. His prints matched those from the crime scene. Ironically, Perugia's thumbprint had been in Bertillon's collection all the time, but it was of his right thumb. The one left on the glass in the Louvre was from his left thumb.

Criminals soon realized that fingerprints could be used to convict them and took evasive measures. Some used gloves but others, like John Dillinger, a gangster who terrorized the Chicago area in the 1930s, went further. While on the run from authorities, he had a plastic surgeon burn off the outer layer of his fingertips with acid, in the belief that this would erase his fingerprints for good. A tip off put the FBI on Dillinger's trail, they confronted him and shot him dead. In the morgue, they discovered Dillinger's attempts to burn away his this fingerprints. He had not succeeded. Fingerprints usually grow back and, in any case, go down through several layers of skin.

Early fingerprint investigators had a tough job sorting manually through print records. Today, matching is accomplished with the aid of high-speed computers. The FBI began to automate print analysis in the 1960s with AFIS , the Automated Fingerprint Identification System . The AFIS computer scans and digitally encodes fingerprint records into a database. It can match a sample, either a ten-print set or a single or partial print, by searching the database. Early versions of AFIS searched hundreds to thousands of prints a second; now the speed is up to 500,000 prints per second.

One notable success for AFIS was catching Richard Ramirez, a notorious killer known as the Night Stalker. He had committed a number of brutal rapes and murders throughout Southern California between 1984 and 1985, entering victims' homes at night and cutting the phone line. He would shoot any men present before raping their spouse, often in the same bed where the corpse laid. His final crime involved a couple in Mission Viejo, where he shot the man and raped the wife. Fortunately, both survived and the woman saw Ramirez' car, while another witness got the number of the vehicle. The stolen car was found abandoned and a partial fingerprint was recovered from the vehicle. The Los Angeles Police Department had just begun to use an AFIS system that could compare more than 60,000 prints per second and they found a match for the print in the car within minutes. A photo of Ramirez, a 25-year-old drifter from El Paso, went out in the papers and he was recognized within a day by residents in east Los Angeles, who overpowered him when he tried to steal another car. He was convicted by a jury and, on November 7, 1989, was given 19 death sentences.

Palm prints contain even more detail on them than fingerprints, and helped solve the kidnap and murder of 12-year-old Polly Klaas in 1993. The girl was enjoying a pajama party with friends at her home in Petaluma, California, when a man appeared through an open window with a knife and carried her off. The FBI used special light sources and fluorescent powder to locate an otherwise invisible palm print on a bunk bed. They also had a description of the intruder from the other girls. Torn children's clothing was found a few weeks later near a site where a man's car had rolled into a ditch. That man was Richard Allen Davis, who had two previous convictions for kidnapping. A fingerprint expert was able to match the FBI's palm print found at the scene of the crime to Davis, who then confessed and showed police where Klaas' body was. He was sentenced to death in 1996 for kidnapping and murder.

see also Anthropometry; Fingerprint; Latent fingerprint; Ridge characteristics.

Fingerprint Analysis

█ AGNIESZKA LICHANSKA

Fingerprints are the patterns on the inside and the tips of fingers. The ridges of skin, also known as friction ridges, together with the valleys between them form unique patterns on the fingers. Fingerprint analysis is a biometric technique comparing scanned image of prints with a database of fingerprints. Uniqueness of prints, and the fact that they do not change during a person's life, form the basis for fingerprint analysis. The uniqueness of the prints is determined by the minute changes in local environment during fetal development; therefore, the identical twins undistinguishable by DNA analysis can be differentiated with fingerprint analysis. Although the fingerprint pattern remains the same, growth accounts for an enlargement of the patterns. Additionally, accidents or some diseases may alter fingerprint patterns

History of fingerprint use. Notes about the ridges, loops, and spirals of fingerprints were first made in 1686 by Marcello Malpighi. However, it was not until 1880 that fingerprints were recognized as a means of personal identification by Henry Faulds, who also identified a first ever fingerprint. The first book about fingerprints was published in 1888 by Sir Francis Galton, and was titled simply Fingerprints. Galton established the first classification system for fingerprints and was the first to assert that no two prints are the same, or that the odds of two prints being identical were about 1 in 64 billion. Later, the Henry Classification System was developed in 1901 by Sir Edward Henry, and today forms the basis for print recognition in most English speaking countries. This system categorized the ridge patterns into three groups: loops, whorls, and arches.

Fingerprinting was soon introduced in prisons, army and widely used for identification by law enforcement. The Federal Bureau of Investigation collection has millions of fingerprint cards and consists of approximately 70 million fingerprints. Although the main use of prints remains in forensic science and law enforcement, new uses of fingerprints have been developed.

Detection of fingerprints. Presence of pores on the surface of the ridges of the fingers results in the accumulation of perspiration on the fingertips. This moisture remains on the surface of the object a person touches, leaving prints. Depending on the surface touched, prints can be visible to the naked eye (e.g. metal, glass or plastic) or invisible (paper, cardboard or timber). Prints left on non-porous surfaces such as metal can be visualized with powders and lifted with tape. In contrast, the prints on porous objects require special lighting, such as lasers or x rays.

There are two major methods of the identification of fingerprints—comparison of lifted prints and live scanning. The first method is mainly used in forensics, while the second is used for authentication purposes (in security applications) and is also slowly becoming a method for identification at some police stations.

Analysis and classification of fingerprints. Ridges present on the fingers are classified based on the patterns they form. The most important features are ridge endings and bifurcations (separation of a ridge into two). These features are called minutiae and form the basis for further classification and identification. Based on the forms created by the minutiae (loops, whorls, etc.) fingerprints are further sub-classified into many more distinct patterns.

Modern fingerprint analysis uses computer algorithms to determine the similarity between a print and images stored in a database. Analysis is usually performed on multiple levels. First, the algorithms are compared to the prints on the coarse level to identify a type of a print, and then subsequently to identify more and more details until a match is found. The computer analysis of prints compares ridges, bifurcations and their relative location. Fingerprint analysis software and scanners identify a set number of similarity points, this number being determined by the software used, typically up to 90 points are compared. After identification of a set number of features, a template of the scanned print is formed and this is subsequently compared to the templates stored in the computer to determine if the print has a match. Although limiting the characteristics to be compared speeds up the matching process, it can also affect the accuracy if inadequate numbers are compared. Accuracy also depends on the application for which the fingerprint analysis is used.

Scanners have comparison algorithms and a number of recognizable characteristics programmed in, together with the prints of the users (enrolment) to provide the templates for comparison. The FBI fingerprint system is over 98% accurate, while the authentication systems accept only 97% of authorized users. Among some of the reasons for the rejection are: scars, calluses, cracks, dirt, or excess fingernail length.

Fingerprint analysis tools. Two types of fingerprint scanners are normally used, optical scanners and capacitance scanners. Optical scanners identify the print using light; depending on the brightness of the reflected light, optical scanners depict ridges as dark and valleys as light. Capacitance scanners determine the print by using an electrical current. Valleys and ridges on the fingers produce different voltage output, allowing for discrimination between them.

As sophisticated they are, the existing scanners are not totally immune to fraud. Optical scanners can be fooled by a picture, whereas the capacitance scanners can be fooled by a mold of a finger. Some scanners also have temperature and pulse sensors, but they are still vulnerable to molds placed over real fingers.

A number of portable fingerprint scanners were developed mainly by computer companies to provide a secure access for the users. In 1998, Compaq was the first to have a print reader attached to the computer. Currently, there are multiple systems for use with desktop and laptop computers in the form of PC cards and biometric mice. A portable print reader used for computer security employs a tiny digital camera to take a picture of a print and convert it into a map that is subsequently stored in the computer and cannot be duplicated.

Commercial fingerprint identification systems were introduced over 15 years ago. They are now used in security applications to gain access to a building or areas within the building, or computers or network access. Some companies, police offices, and high-security government buildings require fingerprint identification for access to the building or its selected parts.

In order to protect sensitive data, some businesses and the military often use scanners that are attached to computers (the U-Match mouse, for example) or installed in keyboards. These provide either immediate identification for access to the terminal or remote identification for access to secure documents or archives. NATO facilities in Turkey, and the U.S. Office of Legislative Council uses similar technology. New scanner trials are on the way to provide the same protection for e-commerce and Internet banking in order to secure transactions.

In order to combat cell phone thefts, the industry is considering equipping phones with fingerprint readers. Fingerprint protection is also offered for a new generation of safes, such as those provided by Biometrics Marketing. Finally, the scanners are being used to replace timecards in companies and to integrate payroll systems. Five U.S. airports, including Chicago's O'Hare have installed finger-print scanners to check employees' backgrounds. Some banks use fingerprint scans before a check is cashed. Similarly, government agencies sometimes utilize fingerprint scans to ensure that payments are given to the proper recipients.

Today, fingerprint analysis technology is the most wide-spread biometric method of identification and authentication for forensic and security purposes.

█ FURTHER READING:

BOOKS:

Ashbourn, Julian. Advanced Identity Verification: The Complete Guide. London: Springer Verlag, 2000.

Nanavati, Samir, Michael Thieme, and Raj Nanavati. Biometrics: Identity Verification in a Networked World. New York: Wiley and Sons, 2002.

ELECTRONIC:

Find Biometrics. <http://www.findbiometrics.com/index.html> (14 December 2002).

NCSC. "Individual biometrics." <http://ctl.ncsc.dni.us/biomet%20web/BMFingerprint.html> (14 December 2002).

SEE ALSO

FBI (United States Federal Bureau of Investigation)
Forensic Science
Identity Theft

fingerprints In the minds of many people, fingerprints are significant only for the identification of criminals. But the skin corrugations that produce fingerprints are a functionally important part of the structure of the finger pads, not only in human beings, but also in a number of other mammals, especially primates, which use prehensile hands (and feet) for active exploration of surfaces and for the fine manipulation of objects.

Fingerprints form naturally during the development of the human fetus, starting about 13 weeks after conception. The inner surfaces of the fingers and the palms of the hands, which are covered with hairless (glabrous) skin, develop tiny ‘pods’, which are the precursors of the pores of sweat glands. These pods, or ridge units, expand and coalesce with neighbouring pods, producing roughly linear ridges, with the sweat pores distributed along their crests, raised above the surface of the surrounding skin. These form the familiar parallel and swirling ridges and intervening furrows, the exact pattern of which is determined by complex, irregular stresses in the skin. They can be seen not only on the pads of the fingers and thumb, but over much of the glabrous skin on the undersurface of fingers and toes, and on the palms and soles of the hands and feet. (A little talcum powder dusted over the surface of the skin makes the pattern more easily visible.)

The ducts of the sweat glands open through the pores on the crests of the ridges. The moistening of the ridges, combined with the texture of the corrugations, increases friction when in contact with objects and hence improves grip. The regions of glabrous skin that have these epidermal ridges are especially richly supplied with cutaneous sensory nerves. These include large fibres that terminate in specialized endings that are sensitive to mechanical stimulation, in particular structures called Merkel's discs and Meissner's corpuscles, which are acutely sensitive to touch and to low-frequency vibration of the skin, respectively. The individual nerve fibres innervating the finger pads branch over areas of skin that are tiny compared with similar classes of fibres in other parts of the body surface. Thus the receptive field of each such fibre in the finger pads (the area of skin over which the application of an appropriate stimulus will cause an individual nerve fibre to respond) can be smaller than 1 mm². The skin of the fingertips therefore excels in its capacity to detect and discriminate the texture and three-dimensional shapes of surfaces. As the fingertips are moved over a non-smooth surface (when a blind person reads embossed Braille characters, for example), the resulting pattern of impulses generated in these nerve fibres and transmitted up to the brain provides remarkably acute tactile perception.

Serendipitously, the sweat secreted onto the skin ridges leaves an oily image of the pattern of corrugations on any surface that is touched. Since the exact forms of the corrugations are unique to each individual, and do not alter from birth to death (unless the skin is badly injured), fingerprints provide an infallible method of identification.

History

The discovery of the uniqueness of fingerprints is very ancient: the Chinese and Assyrians used them on legal documents more than 2000 years ago. The great Czech anatomist, Jan Evangelista Purkinje (who gave his name to the main class of nerve cells in the cerebellum), studied the pattern of skin ridges and, in 1823, suggested a method of classification. The first Police Fingerprint Bureau was established in Argentina early in the 1890s by a Croatian immigrant, Juan Vucetich, whose system of classification is still used in South America. But the best-known system was devised a few years later by a police official, Khan Bahudur Azizul Haque, of the Bengal police, under the direction of Edward (later Sir Edward) Henry. When Henry was appointed Assistant Commissioner of Police for the Metropolis in London at the turn of the twentieth century, he established the first British Fingerprint Bureau at New Scotland Yard. The Henry System, based on prints of all the fingers, spread throughout the world. It quickly replaced the Bertillon system for identification, devised in 1879 by the French criminologist Alphonse Bertillon, which involved anthropometric measurement of parts of the body and detailed records of personal features, such as scars and eye colour.

The appearance of fingerprints

A record of an individual's fingerprints is made by inking the pads of the fingers and thumb and pressing or rolling them onto paper or some other suitable material. The most distinctive overall fingerprint patterns (arches, loops, and whorls) occur on the pads of the fingers, thumbs, and toes. The one illustrated is a magnified image of a ‘loop’, the commonest such pattern, which almost everyone has at some point on their skin. The sweat pores appear as little white dots at intervals along the ridges. Within such overall patterns are individual features known as ‘ridge characteristics’, ‘ridge detail’, or ‘minutiae’. In places, the parallel ridges split into two (a fork or bifurcation), stop dead (a ridge ending), or divide into two and then join up again (a lake or enclosure). These features were studied and named by the British anthropologist and founder of the science of eugenics, Sir Francis Galton. It is such features (marked in the figure) and their proximity to one another that define the unique individuality of fingerprints. The prints of each finger pad typically contain about 100 such minutiae.

In many people, the overall macro-patterns on corresponding fingers of the left and right hands are roughly mirror images of each other. But it is possible to have a different feature pattern on every digit. Although the gross patterns (arches, loops, and whorls) may be passed on through family lineage, the individual ridge details are not. Since these minutiae are presumably not directly genetically determined, they differ between identical twins, and presumably would not be the same even in the clone of an individual.

Minor cuts and abrasions, and some skin diseases (e.g. psoriasis and eczema), may temporarily disturb the ridged skin features, but after healing the structure is exactly the same as before. More serious injuries or burns, involving the deeper layer of the skin (the dermis), can damage the cells that are responsible for regeneration of the skin, and leave a scar within which the ridge pattern is lost or changed. However, identity can still be established from surviving features outside the area of damage. A notorious American criminal, Roscoe Pitts, eliminated the prints of his fingertips by having them sewn into incisions in his chest until new skin grew over the ridge pattern. But he was later identified from prints of his palm, left at the scene of a crime!

Use in crime detection

When a surface is touched by a human hand, oily sweat is deposited from the skin ridges. This ‘latent print’ is often invisible to the naked eye, but can be revealed by a light dusting of powder, using a fine brush. Nowadays, aluminium flake powder is usually applied with a fibreglass brush, but a whole battery of physical and chemical processes is also available to develop prints on almost any surface.

Fingerprints collected at the scene of a crime are then matched against those taken from previously convicted criminals (now held as graphics files on computer databases), or from suspects. Although there have been encouraging developments in automated, ‘expert system’ computerized methods for the matching process, identifications are still made exclusively by experienced Fingerprint Officers.

Fingerprints are not the only means of identifying individuals. Voiceprints, lip prints, ear prints, glove prints, and DNA profiling provide additional or alternative methods for recognition.

S. E. Haylock

FINGERPRINTS

Impressions or reproductions of the distinctive pattern of lines and grooves on the skin of human fingertips.

Fingerprints are reproduced by pressing a person's fingertips into ink and then onto a piece of paper. Fingerprints left on surfaces can be obtained and examined through a dusting process and other processes conducted by forensics experts.

The lines and grooves in fingertips are unique personal characteristics, and thus no two persons have identical fingerprints. Although various scientists had earlier observed the intricate and varying patterns of fingerprints, their use as evidence in trials is undocumented in Anglo-American law before the nineteenth century. In 1880 Henry Faulds, a Scottish physician, suggested in a letter to the British journal Nature that fingerprints could be used for identification purposes in a criminal investigation. Courts in the United States began to accept fingerprints as identification evidence in legal cases in the early twentieth century.

Fingerprints may be used in both civil and criminal courts when they are relevant to a case. They are most common in criminal prosecutions, where they may be used to identify the defendant and connect the defendant to the crime. In a murder prosecution, for example, the defendant's fingerprints on the murder weapon may be offered as evidence tending to show that the defendant committed the crime.

The taking of fingerprints from a criminal defendant raises no fifth amendment concerns. Under the Fifth Amendment to the U.S. Constitution, no person may be compelled to be a compulsory witness against himself or herself. However, this provision generally applies only to involuntary confessions and forced testimony. A person suspected of a crime does not have the right to be free from the taking of fingerprints. Criminal suspects may also be required to surrender other personal information, such as physical appearance and measurements, handwriting and voice samples, teeth bites, normal walking gait, and normal standing posture. Unlike most of these characteristics, fingerprints cannot be easily changed.

Fingerprints are also used outside of court for a variety of purposes. Federal, state, and local lawmakers use them to help manage government resources. For instance, many states fingerprint the recipients of public assistance to ensure that only qualified recipients receive assistance. In many jurisdictions a set of fingerprints or a thumbprint is taken from a person who is arrested and then released before her or his court date. This gives law enforcement authorities an identifying characteristic to use in apprehending the defendant in case the defendant does not appear in court for the prosecution.

In Georgia, liquor manufacturers, distributors, wholesalers, and retailers must send a set of fingerprints to the Georgia Bureau of Investigation when they apply for a license to conduct business in the state. The fingerprints are checked against those of convicted criminals as part of a background check on the applicant (Ga. Code Ann. § 3-3-2 [1996]).

Fingerprint information is easily accessible to police departments across the United States. Under 28 U.S.C.A. § 531 (1996), Congress appropriates funds for the creation and maintenance of a national computer database containing the fingerprints of convicted criminals and former criminal suspects. The database is called the Integrated Automated Fingerprint Identification System. Any state that requires persons convicted of sex offenses to submit DNA samples qualifies for the funding and federal support needed to implement the system.

DNA fingerprinting, or profiling, identifies the chemical pattern in an individual's genetic material. It is a very complex analysis. Nevertheless, it is widely accepted by courts in the United States and generally is considered to yield results that are as accurate as those of regular fingerprinting.

There has been some recent controversy over the admission of fingerprints in criminal cases. At least 40 challenges have been filed against the admission of fingerprints in courts, most of them in the past 10 years, and one was upheld. U.S. District Court Judge Louis Pollak ruled against the use of fingerprints in a murder trial in Philadelphia in 2002, but reversed himself two months later. A book questioning the reliability of fingerprints, Suspect Identities: A History of Fingerprinting and Criminal Identification by Simon Cole, was published in 2001.

further readings

Aitken, C.G.G. 1995. "Evaluating DNA Evidence for Identification." Southern California Interdisciplinary Law Journal 4.

Cohen, Peter J. 1996. "How Shall They Be Known? Daubertv. Merrell Dow Pharmaceuticals and Eyewitness Identification." Pace Law Review 16.

Cole, Simon. 2001. Suspect Identities: A History of Fingerprinting and Criminal Identification. Cambridge: Harvard Univ. Press.

Killerlane, James J. 1995. "Finger Imaging: A Twenty-first Century Solution to Welfare Fraud at Our Fingertips." Fordham Urban Law Journal 22.

Mrowka, Molly J. 1996. "Criminal Procedure: Identification by Use of Thumbprints." McGeorge School of Law, University of the Pacific 27.

Vigoda, Ralph. 2003. "Fingerprints Put To Test." Philadelphia Inquirer (January 28).

cross-references

DNA Evidence; Forensic Science.

Fingerprint

Fingerprints are the impressions that are left behind by tiny ridges in the skin on the tips of the fingers and on the palms of the hand. The patterns left by these ridges, which are called friction ridges, are unique to every person. They are determined by the time a fetus is about six months old and they remain constant throughout a person's life. Even identical twins with identical DNA have different patterns of friction ridges on their fingers. Although many features of a person can be changed, fingerprints cannot. As a result, fingerprints are an extremely important tool for identification of individuals.

The outer layer of skin contains many microscopic pores that secrete sweat and oils. Sweat is mostly water, but it contains a very small fraction (1.5%) of salt, amino acids, and proteins. These chemicals remain on the skin after the water evaporates. The skin also contains sebaceous glands, which produce oils. Although the fingertips contain few sebaceous glands, the face and head contain many and people touch their faces and hair often, transferring oil to the fingertips. The oil and the residual chemicals from sweat cling to the surface of the fingers and attract dirt and other substances such as cosmetics and grease from foods and oils. Whenever a person touches something, these residues are transferred to that surface. Since people rarely commit crimes without using their hands, the prints from their fingers are often left on surfaces at the crime scene.

Detectives look for fingerprints at crime scenes in locations where things have been broken or disturbed. They also usually check the doorknobs and doorways, where a criminal may have entered or exited. Fingerprints can be found on a variety of surfaces including paper, human skin, smooth surfaces, painted surfaces, glass , the insides of gloves and firearms . They can last for a just a few hours in cold, dry weather or they may be visible indefinitely in warm, moist environments.

Fingerprints are classified into three groups. Plastic prints are prints that make an impression on a pliant surface like putty or tacky paint. Visible prints occur when someone has a material on their fingers that leaves a visible mark, such as blood , ink, or make-up. The most common fingerprints are called latent prints and they are formed from the oils and residues on the hands. Latent prints must be developed using one of many different chemical techniques.

Dusting for fingerprints is the most common technique for visualizing a latent print. This process begins by dipping a very soft brush into very fine powder. Most fingerprint kits contain black, gray, white, and red powders and the detective will choose a color of powder that contrasts best with the surface on which the print has been left. The detective carefully brushes the powder over the print and then blows the excess powder away. After the print becomes visible, it is photographed and then transferred onto special tape in a process called lifting.

Several other chemicals and techniques are commonly used to develop latent prints, and they are chosen depending on the surface and other environmental conditions. A chemical called ninhydrin, which is attracted to the amino acids that remain on the skin after the water in sweat evaporates, is used to develop fingerprints on paper. Iodine fumes can also be used to develop fingerprints on paper. The iodine vapors react with oils, turning them a brownish-violet color. Surfaces containing fingerprints can be dipped into or sprayed with silver nitrate, which turns black in the presence of salt. Superglue^TM fumes, which produce white crystals in the presence of moisture in the fingerprints, are also commonly used to develop latent prints. In addition, specialized light sources, such as lasers and ultraviolet lights, can be used to make latent prints appear in situations where chemical techniques are impractical.

There are three basic classifications of fingerprints: arches, loops, and whorls. Of these, loops are by far the most common, next are whorls and a small fraction are arches. Arches are classified into plain arches, which are generally symmetric arched friction ridges, and tented arches, which become so narrow that their core is a single friction ridge. Loops look somewhat like a cursive letter "e," but can be slanted either to the right or to the left. Loops are subdivided into radial loops, which flow towards the thumb, and ulnar loops, which flow toward the little finger. Whorls are circular or spiral shapes. They are subdivided into plain whorls, double loop whorls, central pocket whorls, and accidentals. As a result there are eight major categories of fingerprint patterns.

Fingerprint experts start with the basic patterns of friction ridges when they study fingerprints, but they depend heavily on the details called minutiae within fingerprints. These minutiae include ridge endings, dots, short ridges, bifurcations, and trifurcations. In addition, the location of sweat pores and the pores for oil glands serve as markers that can be used for identification.

see also Bloodstain evidence; Crime scene investigation; Fingerprint; Superglue® fuming.