GAINESVILLE, Fla. — The Kemp’s ridley sea turtle lay belly-up on the metal autopsy table, as pallid as split-pea soup but for the bright orange X spray-painted on its shell, proof that it had been counted as part of the Gulf of Mexico’s ongoing “unusual mortality event.”
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Under the practiced knife of Dr. Brian Stacy, a veterinary pathologist who estimates that he has dissected close to 1,000 turtles over the course of his career, the specimen began to reveal its secrets: First, as the breastplate was lifted away, a mass of shriveled organs in the puddle of stinky red liquid that is produced as decomposition advances. Next, the fat reserves indicating good health. Then, as Dr. Stacy sliced open the esophagus, the most revealing clue: a morsel of shrimp, the last thing the turtle ate.

“You don’t see shrimp consumed as part of the normal diet” of Kemp’s ridleys, Dr. Stacy said.

This turtle, found floating in the Mississippi Sound on June 18, is one of hundreds of dead creatures collected along the Gulf Coast since the Deepwater Horizon oil rig exploded. Swabbed for oil, tagged and wrapped in plastic “body bags” sealed with evidence tape, the carcasses — many times the number normally found at this time of year — are piling up in freezer trucks stationed along the coast, waiting for scientists like Dr. Stacy, who works for the National Oceanic and Atmospheric Administration, to begin the process of determining what killed them.

Despite an obvious suspect, oil, the answer is far from clear. The vast majority of the dead animals that have been found — 1,387 birds, 444 turtles, 53 dolphins and one sperm whale — show no visible signs of oil contamination. Much of the evidence in the turtle cases points, in fact, to shrimping or other commercial fishing, but other suspects include oil fumes, oiled food, the dispersants used to break up the oil or even disease.

The efforts to finger a culprit — or culprits — amount to a vast investigation the likes of which “CSI” has never seen. The trail of evidence leads from marine patrols in Mississippi, where more than half the dead turtles have been found, to a toxicology lab in Lubbock, Tex., to this animal autopsy room at the University of Florida in Gainesville. And instead of the fingerprint analysis and security camera footage used in human homicides, the veterinary detectives are relying on shrimp boat data recorders and chromatographic spectrum analysis that can tell if the oil residue found in an animal has the same “chemical signature” as BP crude.

The outcome will help determine how many millions BP will pay in civil and criminal penalties — which are far higher for endangered animals like sea turtles — and provide a wealth of information about the little-known effects of oil on protected species in the Gulf.

“It is terribly important to know, in the big scheme of things, why something died,” said Moby Solangi, the director of the Institute for Marine Mammal Studies in Gulfport, Miss., where the initial turtle necropsies and some dolphin necropsies were performed.

“We might be doing what we can to address the issues of today and manage the risk,” he said. “But for tomorrow, we need to know what actually happened.”

Searching for a Smoking Gun

In a laboratory at Texas Tech University in Lubbock, Jennifer Cole, a graduate student, was slicing a precious chunk of living dolphin tissue into .3-millimeter sections.

Supervised by Céline Godard-Codding, an endangered species toxicologist, Ms. Cole was studying cytochrome P450 1A1, an enzyme that breaks down hydrocarbons.

Tissue samples are one of the only ways to learn more about toxins in marine mammals and sea turtles, whose protected status limits the type of studies that can be done — researchers cannot do experiments to determine how much oil exposure the animals can withstand.

Oil — inhaled or ingested — can cause brain lesions, pneumonia, kidney damage, stress and death. Scientists working on the BP spill have seen oil-mired animals that are suffering from extreme exhaustion and hyperthermia, with the floating crude reaching temperatures above 130 degrees, Dr. Stacy said.

Far less is known about the effects of dispersants, either by themselves or mixed with oil, though almost 2 million gallons of the chemicals have been used in the BP spill.

Studies show that dispersants, which break down oil into tiny droplets and can also break down cell membranes, make oil more toxic for some animals, like baby birds. And the solvents they contain can break down red blood cells, causing hemorrhaging. At least one fresh dolphin carcass found in the Gulf was bleeding from the mouth and blowhole, according to Lori Deangelis, a dolphin tour operator in Perdido Bay.