This is what you're going to get from the medical examiner and it is his/her best guess for the present situation.
This marks the time when the victim's vital organs for sustaining life cease to function.
The basic time of death estimation should be given when and where the body is discovered, before it gets photographed, moved, and transported. A medical examiner (or coroner, depending on your jurisdiction) will try to “guesstimate” the approximate time the person died based on several factors. In her article for Crime Library, Katherine Ramsland tells us that time of death is one of the central factors in any murder case because it can eliminate suspects, break alibis, or place victims clearly with the suspect. That's a given. Not to mention that it helps to form a timeline, giving detectives important clues as to what could have happened when. Time as an element in an investigation has the greatest potential for leads, other than a confession of course. People usually run their lives on routine, so by figuring out how long they've been dead, helps an investigator to question friends and family; in essence, he asks: Where should they have been? Doing what? Why? With who else?
Here is a short descriptive list of what is looked at in part or in whole to help determine time of death:
1. Body temperature, also called algor mortis—a body cools when oxygen no longer is being pumped through to help keep a normal body temp of 98.6 degrees Fahrenheit. The rate is approximate, but a rectal temperature reading is divided by 1.5, which is equal to one to one and a half degrees per hour since death, when the body takes on the temperature of its surroundings.
2. Discoloration from livor mortis, also referred to as lividity or hypostasis—in a dead body the fluids settle, and a dark purple color is found on the lower side where it is lying. It happens because the white and red blood cells don’t mix anymore and the reds are settling. If part of the body is lying on a grate, that pattern will be repeated on the tissues because the contact makes pressure and keeps the blood from pooling there. A phenomenon called “blanching” (press your finger to your skin, it leaves a white trail) can indicate that lividity is not permanent, and that death is probably more than two hours old but not as long as ten. Two factors may change blood color, carbon monoxide poisoning (presents a bright red color) and extreme loss of blood (no discoloring at all).
3. Food digestion—examination of stomach contents can time date when someone has eaten their last meal, and the death estimation is based on the fact that the stomach breaks down food and empties it into the intestines at a fairly predictable rate. There are many factors that can play into this determination, however, including metabolism, drugs, medication, presence of disease, and the person’s emotional demeanor.
4. Rigor mortis—this action is due to muscles stiffening on account of accumulating waste products. It generally begins a couple to three hours after death and seems to start in the face, lower jaw and neck, and then spreads over the next twelve-plus-hours up to about eighteen hours to the larger muscles in the body. The rigidity can last up to thirty-six hours after which it then begins to disappear, starting at the head and face again. This process too, is affected by temperature (heat speeds the process) and differences in musculature (if not much muscle, less stiff).
5. Decomposition and decay—a dead body builds up fluids and gas due to the microorganisms that live inside. Liquids can emanate from the mouth and nose, the tongue swells, and the abdomen turns a greenish-yellow color. Over time, the skin will blister and fill with fluid. The decomposition process is sped up by heat and slowed under colder conditions. "Decomp" attracts insects as well and they set about the body to feed and lay eggs (usually in the bloody areas first). Their larval stages are what help to predict death. Bare in mind, Dear Reader, that forensic entomologists are a rare species themselves because there are probably no more than seventy certified forensic professionals in the whole of the country.
6. Ocular—eyes that are left open after death will develop a film on the surface. The red blood cells break down here too (in the “vitreous humor”), and the eyes after two or three hours will appear cloudy. Cloudiness shows in closed eyes as well, but much more slowly.
7. Personal factors—people are a great source of time death estimation as in “the last time I saw so-and-so . . .” ; those missing an appointment help to pinpoint time, a broken watch or timepiece will help, or a time-date from a camera are other considerations.
8. Other scientists—a forensic botanist may be able to give time-date clues based on the presence of pollen, pods or whatever vegetation growth or stunting may have occurred beneath or around the body; forensic anthropologists can help add notes after skeletonization, and toxicologists will have something to say about the fluids and blood.
Time of Death, Decomposition and Identification: An Atlas by Jay Dix, M.D. and Michael Graham, M.D., CRC Press, January 2000. ISBN: 0-8493-2367-3
Forensic Pathology by Dominick J. DiMaio, M.D. and Vincent J.M. DiMaio, M.D., CRC Press, June 2001, ISBN: 0-8493-0072-X
Corpse: Nature, Forensics, and the Struggle to Pinpoint Time of Death by Jessica Snyder Sachs, Perseus Publishing. (Have not reviewed as yet.)