Knoxville, Tenn. - Under different circumstances, it would have been an unforgettable crime scene. The warm autumn sunshine filtering through a red- and orange-hued maple. The quiet wooded site by the Tennessee River. The forty or so bodies in various stages of decay. But as a small group of slightly anxious journalists would soon discover, the donated human corpses had been purposefully placed here after deaths attributable to accidents, to cancer, to old age. Hidden from public view by a 9-foot wooden privacy fence ringed by a razor wire-topped chain-link fence, they would not, however, rest in peace.

Even in death, they still had work to do.

Over days, weeks and months, the bodies would succumb to the inevitable process of nature reclaiming her own, with a progression of assisting insects, bacteria and the occasional raccoon or opossum. And here, within the University of Tennessee's Anthropology Research Facility, scientists would record the unsavory details in an effort to answer the crucial questions faced by crime scene investigators hoping to bring a measure of justice to bodies felled by more sinister motives.

Here on what they call the Body Farm.

"If you think about it, this is how nature disposes of bodies, and we're not different from anything else," said Richard Jantz, director of the university's Forensic Anthropology Center, during a tour of its two-acre, one-of-a-kind research compound.

Founder and former director Bill Bass oversaw the facility's creation more than 20 years ago in response to a host of unanswered questions. How, he and other researchers wondered, could they more accurately estimate time since death? How many identifying details could be extracted from decomposing remains? How could investigators find hidden graves, a question that has since found international relevance in places like Bosnia, Rwanda and Iraq?

A growing number of answers have emanated from the burgeoning discipline of forensic science, which has figured prominently in television shows such as "CSI," in novels such as "The Body Farm" by mystery writer Patricia Cornwell, and in a new historical account of the research facility by Bass and co-author Jon Jefferson, titled "Death's Acre."

Some of the 30 to 50 cadavers arriving at the Body Farm each year come courtesy of local medical examiners donating unclaimed bodies. But much more frequently, the arrivals are prearranged by consenting donors who have expressed an active interest in the facility's research and who have completed a biological questionnaire detailing their medical histories.

The facility has amassed hundreds of these completed questionnaires by its future donors.

During their talks at a conference held by the Council for the Advancement of Science Writing, Jantz and fellow researcher Arpad Vass detailed the clues to be gleaned from nature's disposal process - a process that begins about four minutes after death.

Each stage includes its own march of the macabre. Flies begin laying their eggs in available crevices during the fresh stage, said Vass, a forensic scientist at Oak Ridge National Laboratory. The gaseous by-products of bacteria lead to bloating during the second stage. In the third, called active decay, the body's soft tissue liquefies and insect holes proliferate. And in the fourth, or dry, stage, the body becomes little more than bones.

The pace of this post-mortem march depends chiefly on temperature, according to Vass, since heat fuels biochemical reactions within the body. But water content, pH levels, and oxygen availability also can influence decay rates and confound scientific estimates.

In "Death's Acre," for example, Bass and Jefferson recount the bizarre Tennessee case of a body that had been partially disinterred in 1977. Bass initially characterized the corpse as being that of a recently deceased man. The time of death estimate, however, was off by nearly 113 years - a blunder that would later spur him to found the Body Farm. (Researchers later discovered that the corpse was that of a Civil War soldier, which had been embalmed and well-preserved within an airtight cast-iron casket.)

As forensic anthropologists are finding, however, the natural processes of decay leave distinctive and surprisingly reliable calling cards.

Even after death, a few of the body's biochemical cycles continue their futile progressions, churning out selected products and by-products as long as the necessary nutrients are available. The Krebs cycle, a cascade of biological reactions long dreaded by any college student forced to memorize it, offers one such deathless progression. The rate of a specific reaction varies from organ to organ, however, leading to measurable differences in the product - glycolic acid - accumulating throughout a decaying body.

In a study published last year, Vass and colleagues measured the abundance of glycolic acid within the liver, kidney, heart, brain and muscle of 18 donated cadavers as they decayed at the Body Farm - just a few of the hundreds of bodies that have been donated over its two-decade history. In combination with other biological markers examined within the first three weeks of body decay, the measured levels of glycolic acid formed a biochemical tick-tock that can estimate time since death to within 12 hours.

To produce estimates of longer than three weeks, Vass turned to other compounds, known as volatile fatty acids, which are formed by the breakdown of fat and protein. Again, he discovered that the compounds produce separate signatures over the course of decomposition. By plotting five of them over time, his team created a timetable that can estimate time since death with an accuracy of plus or minus two days for every month of decay.

The relative abundance of inorganic elements released by a body for up to several years helped the team create a third time-since-death template that is accurate to within three weeks per year.

The new timetables haven't yet been widely used at crime scenes. But Dr. Vincent Stefan, a professor of forensic anthropology at Lehman College in Manhattan, said a better understanding of human remains over the past few decades has refined the available set of forensic tools. Stefan, who consults for the Nassau, Suffolk and Westchester county medical examiner's offices, has put such tools to use in an estimated 30 to 50 forensic cases in the New York metropolitan area, including his recent examination of a decomposing body discovered near Manorville.

Although he declined to discuss details of the investigation, Stefan said in a telephone interview that a main goal in forensic cases is to develop a biological profile of the victim using available bones and tissue.

"If you're talking about sex, the pelvis is the most important," he said. "If you're talking about things like race, then the skull would be the most important. If you're talking about stature, you'd like to have the bones of the leg."

Examinations suggest that the Manorville body is intact, but Stefan said investigators don't always have that advantage in establishing identity, uncovering signs of injury, or estimating time since death. "You make do with whatever happens to be found and brought in," he said.

Sometimes, the most important details emerge from the least pleasant aspects of animal and insect activity. At the Body Farm, Walter Klippel led his visitors to the far side of the compound, where he said researchers were studying something euphemistically called animal taphonomy.

"Things that go bump in the night," said Klippel, a zooarchaeologist who normally specializes in studying animal remains.

The full meaning of his unusual research project became evident as he paused before a human corpse, its right arm and leg extending from beneath a black tarp buzzing with flies. Klippel lifted the tarp and pointed out an obvious and odiferous example of raccoon damage to the newly exposed left foot.

More signs of recent animal activity were apparent in the nearby soil, where raccoons had foraged for a fresh meal of maggots. Muddy paw prints trailed across a body bag further up the hill.

Klippel said researchers at the Body Farm were keen to capture such nocturnal disturbances with an infrared camera outfitted with a motion detector. On a warm, damp night, he said, the detector may get tripped as many as 100 times.

Understanding animal and insect activity can provide forensic sleuths with vital clues at a crime scene. Flies are invariably among the first to arrive and the subsequent emergence of their larvae and pupae can help scientists estimate time of death by using the fly's normal two-week life cycle as a clock of sorts. As related in "Death's Acre," this clock helped to convict a man of murdering his stepson's family in a Mississippi cabin.

And by understanding marks left by animals, researchers can separate post-mortem damage from injuries sustained during the murder itself.

"We're always interested in separating human action from animal action," Jantz said.

Graduate student Jennifer Synstelien showed some camera footage of the latter, an opportunistic raccoon pawing at the edges of a white body bag for maggots trapped in the plastic creases. She said animal taphonomy research had focused on damage from canids such as dogs, wolves and coyotes, but nothing had been reported about the effects of smaller scavengers like raccoons, opossums and rats.

On the other side of a leaf-strewn path, a woman's animal-ravaged body lay near an abandoned reddish trailer, as if the former occupant had perished within sight of home. Synstelien said the body was being used for an exploratory project to create a color guide for coroners.

Asked whether the sights ever bothered her, Synstelien merely shrugged, replying that she liked being in the quiet research lot, close to nature.

And the smell?

"I don't think a person ever gets used to the smell," she said.

Decaying human protein eventually yields the compounds cadaverine and putrescine, both of which are readily apparent to the human nose in high concentrations - and unpleasantly so, as their names suggest. In low concentrations, however, scientists believe the odors are perceptible only to flies and dogs.

Trained cadaver dogs, the gold standard in searching for corpses, have in rare cases discovered hidden graves up to 100 years old. But Vass said the reliance on dogs for forensic work is costly, time-consuming and imprecise, especially since researchers have yet to determine what odor combinations are required for canine detection.

The problem is further compounded by a scarcity of well-trained cadaver dogs in the United States, Vass said. "Five have been shown, to my knowledge, to be really good at it," he said.

Some of his emerging research hints at new solutions, or at least at new assistance for search dogs.

Historically, scientists believed that decaying bodies released only a few ephemeral compounds, apart from cadaverine and putrescine, such as methane, ammonia, carbon dioxide and hydrogen sulfide. But Vass and colleagues have pushed the number of known compounds to about 450. That discovery is enabling a soon-to-be-released database dryly called Decompositional Order Analysis, and wryly abbreviated D.O.A.

Vass said 42 of the chemicals are produced in significant quantities, and he hopes to define a subset that is released in most or all instances of decomposition - a step that could help standardize the training of cadaver dogs. But going further to develop an artificial sensor capable of detecting the chemical signature of decay would likely require years, he said.

Vass has several other potential assists in the works. One envisions a grave-detection system in which scientists would isolate bacteria that respond to grave-specific compounds. By engineering the bacteria to contain a jellyfish-derived protein, called green fluorescent protein, scientists could create a bacterial beacon that glows whenever the bacteria encounter the telltale grave compounds.

Researchers could use a sprayer or even a crop duster to apply bacteria to a suspected site, and return with a sensitive fluorescence detector a few days later. That's the idea, anyway. Although a usable field tool is doubtless years away, the team has completed a proof-of-principle experiment with a piece of buried meat.

The key to any criminal investigation, Vass said, is providing forensic sleuths with a variety of reliable and rapid tools.

"I want the crime scene investigator to go to the crime scene and solve that crime in real time," he said.

Many of the scenes have already been simulated within the Body Farm, where researchers have examined cadavers left in pools of water, on a cement slab, and even in the trunk of a junked car.

For his latest biochemical research, Vass buried four donated corpses in separate graves near the facility's entrance. A quick inspection of the first grave revealed little except for three small stainless steel tubes protruding through the soil.

As nature reclaims her own, these tubes will serve as conduits, collecting a host of chemical clues that might otherwise go unnoticed as they slowly wend their way through the red Tennessee clay.

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