Time of Death

The determination of time of death is of crucial importance for forensic investigators, especially when they are gathering evidence that can support or deny the stated actions of suspects in a crime. The time elapsed from the moment of death until a corpse is discovered is also known as the postmortem interval, or PMI.

Both the time of death and the postmortem interval cannot be determined with 100% accuracy, particularly when a body is found in advanced state of decomposition or is recovered from fire, water, or ice. Therefore, time of death and PMI are given as estimates, and can vary from hours to days, or from months to years, depending on each particular case.

Evidence for estimating time of death includes physical evidence present in the corpse (postmortem changes, presence of insects, etc.), environmental evidence such as location where the body was found (indoors, outdoors, buried, burned, in water, etc.), and other evidence found at the crime scene (a stopped wrist watch due to a blow or impact, an answering machine record, a 911 call, phone calls received or made around the time of the assault, etc.), and finally, the historical evidence (habits and daily routine of the victim, relationships, existence of enemies, etc). The knowledge of the internal sequential changes a dead body undergoes in relation to the variations on the rate of their occurrence due to ambient temperature, humidity, and the presence of insects or other predators are all considered when estimating the time of death.

The classical method of estimating time of death is the rate method, which measures postmortem (after death) stages and the types of transformation a body undergoes such as cooling rates (algor mortis), stiffening (rigor mortis ), initiation and duration, postmortem lividity (discoloration stains), degree of putrefaction, adipocere (body fat saponification), and maceration (tissue softening due to the presence of liquid). Not all these stages take place in a single cadaver. Adipocere, for instance, is not common in most male adult corpses. It occurs most often in women or obese adult individuals and children, requiring enough humidity or the presence of water to take place. The process of maceration occurs at known rates in fetuses that died in the womb. Stomach contents can reveal the stage of digestion of the last meal at the time of death. The time of onset and rates of each postmortem transformative event are also subjected to variations originated by existing chronic diseases, types of medication, and individual metabolic characteristics. These variables are known as endogenous factors. For example, if the deceased individual was taking antibiotics at the time of death, the internal process of bacterial-mediated putrefaction may be delayed beyond the normal observed rates, thus masking the real PMI.

Algor mortis, or the process of body cooling, is a useful parameter for PMI estimation during the first 24 hours after death, as the internal body temperature drops at known rates. However, these rates are valid only in cool or temperate climates because hot summer seasons or tropical temperatures slow down the loss of heat and, in some regions, can even raise postmortem temperatures due to rapid putrefaction through bacterial activity inside the digestive tract. Algor mortis rates are measured with a thermometer or through the use of a multiple-probe thermometer that measures the cooling rate of the brain, liver, and rectum. Other variables interfering with postmortem cooling rates include the size of the body, amount of subcutaneous (under the skin) adipose (fatty) tissue, existence of clothing and coverings, air currents and humidity, and the medium where the body remained after death (such as inside a closed car, under water, on ice or snow, or inside a metallic container).

Rigor mortis, or postmortem stiffening and contraction of all muscles, usually occurs three or more hours after death and can last for approximately 36–48 hours in temperate climates and about 9–12 hours in tropical temperatures. If a murderer moves a body before rigor mortis (RM), the new position will be "frozen" during RM, not the original one that would have characterized the pattern of the body falling at the crime scene. Therefore, the position a body shows during rigor mortis cannot be assumed as the position in which the victim was at the moment of death. The rigor mortis phase is not the best time for the pathologist to determine the cause of death , because several changes take place in the internal muscles, such as the heart and the ocular muscles, which can be misleading. For example, rigor mortis dilates the myocardial (heart) muscles, giving it the appearance of cardiac hypertrophy (enlarged heart). Contraction of the iris muscles also dilates the pupils during rigor mortis.

The factors that interfere with the onset and duration of rigor mortis are temperature, existing antemortem pathologies, age, body muscular mass, and the degree of muscular activity immediately before death. Higher temperatures shorten the time till the onset of rigor mortis and its time of duration. A strong fight or lengthy physical effort before death causes an earlier onset and shorter duration of rigor mortis. Children and older adults have also earlier rigor mortis than younger adults. Generalized infections, or long, debilitating diseases also produce earlier onsets and shorter periods of rigor mortis, whereas extensive antemortem bleeding or death due to asphyxia delays rigor mortis onset.

Livor mortis, or postmortem lividity, is characterized by the reddish/purple discoloration of the skin, sometimes with a pink border, in consequence of the lack of the arterial pressure that counteracts the gravitational force. Therefore, when blood circulation ceases, the blood is gradually deposited in the lower internal vessels and in the lower parts of the body, with the signs of livor mortis usually appearing within the first hour after death. However, in many cases it can appear 2–3 hours after death, and is usually fixed after about 12 hours. Livor mortis rates of appearance are delayed by severe anemia and starvation, but can be present before death in individuals slowly dying from circulatory insufficiency.

Postmortem decomposition or putrefaction consists of the destruction of soft tissues, usually starting internally through the action of microorganisms present in the stomach and bowel and in the nasal pharyngeal pathways. Open wounds also provide access to bacteria from the environment to the body. Obesity accelerates the putrefaction process, as well as infectious conditions, congestive cardiac failure, or when edema (swelling with fluid) is present. Conversely, extensive external bleeding during death or severe dehydration delays the putrefaction onset. As mentioned before, temperatures may accelerate or delay putrefaction onset and rates. Gases derived from the putrefaction process are used to estimate time of death, known as the Brouardel method. According to this approach, in the first postmortem 24 hours, abdominal gases are not flammable; between the second and the fourth day they are flammable; from the fifth day on, they are not flammable again. Putrefaction stains start to form on the abdominal skin around 24–36 hours after death in temperate climates and in 12–18 hours in tropical regions. These stains are green and gradually appear all over the body between the third and the fifth day after death. As the blood undergoes putrefaction, crystal blades are formed in fragmented or clustered patterns, crisscrossed and colorless. These crystals start forming after the third day and can remain in the blood up to 35 days. Determining time of death by observing blood crystals is known as the Westernhoffer-Rocha-Valverde method, and was first applied in forensic medicine by the Brazilian forensic pathologists Martinho da Rocha and Belmiro Valverde.

The first postmortem transformative event, known as autolysis, consists of spontaneous selfdestruction of tissues by enzymes present in the cells without bacterial interference. One of the byproducts of autolysis is the building up of potassium ions concentrations known as vitreous humor potassium (VHP), and occurs during the first 20 postmortem hours. The quantitative analysis of the concentration rates of VHP is one of the methods for PMI estimation, which yields the best results when combined with other measurements.

Postmortem tissue survival rates constitute another PMI estimation method. Different types of tissues lose their vital properties in different moments of the postmortem interval. For instance sperm cells show mobility for about 36 hours after death. Muscles react to electrical or mechanical stimuli for a postmortem interval of six hours, and pupils can be dilated with atropine until four hours after death. Leukocytes, the white blood cells, die at the following PMI rates: 8% during the first 5 hours; 58% within 30 hours; and 95% within 70 hours.

Corpses exposed to outdoor environments attract insects with different behavioral habits and life cycles. Another modern technique utilized in time of death estimation involves forensic entomology . Forensic entomology utilizes insects on or surrounding the body, as well as their eggs and larvae, to estimate the amount of time a body has been dead and left in a certain environment. Entomology is useful as a forensic tool because the life cycles of insects are both well known and predictable. In addition, the succession of colonization of a corpse by insects occurs in temporally specific waves of different species.

Once a person or animal has died, insects that have access to the corpse colonize it very rapidly. The succession of inhabitants in terms of species and life cycle stage is clearly understood. This succession can then be used to determine several aspects of the crime. These include post-mortem interval, location of the murder, where the body was stored, and whether or not it had been moved.

The first insects to approach and colonize a dead body are usually species of blowfly (Diptera: Calliphoridae ) or the flesh fly (Sarcophagidae ). These holometabolous insects quickly deposit their eggs on an exposed corpse, and maggots, the larval form, are often found feeding on dead bodies. A forensic entomologist would be able, with the use of a microscope, to identify the stage of a blowfly larva. There are three larval stages, called instars, and by looking closely at the mandibles (mouthparts), genitalia, and spiracles (holes and tubes for gas exchange) the entomologist can differentiate not only the species of the larvae, but also determine whether it is a first, second, or third instar.

The maggots then mature to the pupal form, which is often found deposited around the body. Forensic entomologists are cautious in scouring the region surrounding a corpse for the inactive pupae. However, if care is not taken by an investigator at a crime scene, the pupa can often be overlooked as it resembles rodent droppings. Once the insect has matured to an adult form, it emerges from the pupae. Empty pupal cases found in the vicinity of a body can also yield clues.

Beetles are generally the next insects to colonize a corpse. Carrion beetles of the order Coleoptera also undergo holometabolous development. Compared to the maggot larvae of flies, which are similar among species, the larval forms of beetle species are very different. In contrast to blowfly larvae, all beetle larvae also have legs, so the two orders of insect larvae are immediately differentiated by their appearance. Beetle larvae can be fat, slender, hairy, and a variety of colors from white to dark brown and black.

Forensic entomologists have been instrumental in solving homicide cases in recent years. Not only can they determine the approximate time of death from the succession of adult insects, larvae, and pupae found on the corpse, but they can also provide information such as if the body was moved. For example, if a body is found indoors, but colonized with insects typically found in a wooded outdoor location, the forensic entomologist would infer that the body had been moved.

In addition, bloodstains found at the scene of a crime can yield clues or confound police. Bloodstains could have been recently deposited, or possibly been there for a period of time from events unrelated to the crime under investigation. New and innovative techniques are now being used to establish time of death and age of bloodstains. These new techniques help forensic scientists and criminal investigators reconstruct more representative crime scenes and more precisely determine time of death.

When the suspected perpetrator of a crime is a relative or friend, crime scene analysis and reconstruction is much more complex. When the crime was committed in the relative or friend's home, it is difficult for investigators to separate evidence temporally since it is likely that the victim was in the environment previous to the crime. For example, if a woman murders her husband in their home, there may be small traces of blood in the house. However, this blood may have been present well before the crime and be totally independent of the events of the crime. Although forensic DNA analysis of the stain would easily identify to whom the blood belongs, this analysis would not provide any clues as to when the stain occurred. Similarly, a bit of blood found in an automobile could suggest a body was transported in a car. If the victim of the crime was a family member of the car's owner, how can it be determined if this blood came from a scratch before the crime was committed? Determination of the temporal events surrounding the deposition of a blood sample could prove crucial to solving a crime.

Often, characteristics of the blood protein hemoglobin such as color and solubility are used as an estimation of bloodstain age. These techniques have their drawbacks, however, as it is often necessary to determine the species from which the blood originated, and often the size of the stain affects the analysis. One new technique which shows potential for forensic analysis of bloodstain age utilizes RNA (ribonucleic acid) in the bloodstain. Although messenger RNA (mRNA) is easily degraded, researchers have found that highly abundant mRNA is detectable over six months following blood deposition. Furthermore, if PCR (ploymerase chain reaction, a DNA amplifying technique) is performed using species-specific primers, one can easily tell the species from which the blood originated.

The three different type of RNA—mRNA, transfer RNA (tRNA), and ribosomal RNA (rRNA)—are known to decay at different rates. Researchers have recently shown that using a ratio of mRNA of a highly abundant gene to that of rRNA, it is possible to determine the age of a bloodstain, because the degradation of the ribosomal RNA is much slower. Forensic scientists first isolate RNA from the bloodstain, then use real-time RT-PCR (reverse-transcriptase polymerase chain reaction) techniques to make DNA copies of the RNA. Real-time PCR provides an amplified DNA copy of the RNA, but still maintains a ratio of the amount of transcript in the reaction at the start of the reaction to that of the RNA at the end. By using primers specific to the RNAs of interest, only those are selectively amplified. Thus, it is possible to compare amounts of two different amplified DNAs that reflect the relative composition of those RNAs in the initial sample.

Although both RNA analysis and forensic entomology are relatively new techniques, they have great possibility for crime scene investigation . Forensic entomology has already proven useful in a variety of cases and, with more basic research, it is only a matter of time before RNA techniques prove equally as useful. As forensic techniques become more and more advanced, criminal investigations will be solved more rapidly and with even greater accuracy.

see also Adipocere; Asphyxiation (signs of); Autopsy; Body marks; Crime scene reconstruction; Death, cause of; Death, mechanism of; Decomposition; DNA fingerprint; Drowning (signs of); Entomology; Hanging (signs of); Lividity; Mummies; Pathology; Rigor mortis; STR (short tandem repeat) analysis; Toxicological analysis.