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 continuing “unusual mortality event.”

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 stinking 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 — 1,866 birds, 463 turtles, 59 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 video 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 0.3-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 poisonous substances 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 two 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.

Investigators plan to take skin and mouth swabs, stomach contents, slices of organ tissue and vials of bile from animals that have died and test them for disease and hydrocarbons, as well as for dispersants, before a final report on the cause of death is written. But no samples have yet been sent to labs, because scientists are still evaluating what type of tests will prove most useful.

Jacqueline Savitz, a marine biologist with Oceana, an ocean conservation group, said there was no excuse for any delay in testing.

“It’s absolutely urgent that it should be done immediately,” she said, because the findings could influence response measures like BP’s experimental use of dispersants underwater.

In the meantime, at places like the Texas Tech institute, the oil spill has set off a mad scramble to fill in the gaps in knowledge. In one laboratory, jars of BP crude in various stages of weathering await analysis to determine their relative toxicity. In another lab, graduate students paint precise amounts of oil on incubating duck eggs. Tanks of fiddler crabs awaited a shipment of Corexit 9500, the dispersant being used by BP in the Gulf.

In the end, Dr. Godard-Codding said, scientists will not find a single smoking gun. The evidence — results of laboratory tests, population counts, assessments of how well oil-drenched animals survive after rehabilitation — will all be circumstantial.

Suspicions Fall on Shrimpers

When Lt. Donald Armes of the Mississippi Marine Patrol heard about the rash of dead sea turtles littering the state’s shores, his first thought was not of oil but of shrimp boats.

“Right off the bat, you figure somebody’s gear was wrong,” he said recently, after patrolling for shrimpers in the Mississippi Sound, a few days before floating islands of oil forced officials to close it. By gear, Lieutenant Armes meant turtle excluder devices, which shrimp trawlers are supposed to have. Without them, trawls can be one of the biggest dangers for turtles, which can get trapped in the nets and drown. The devices provide an escape hatch. Another kind of shrimp net, called a skimmer, is not required to have an excluder device — instead, the length of time the skimmers can be dragged is limited by law to give trapped turtles a chance to come up for air.

When shrimp season began in Mississippi on June 3, the marine patrol inspected all the boats and found no violations involving the excluders, Lieutenant Armes said. But on June 6, 12 dead turtles were found in Mississippi in a single day. Similar spikes have occurred when parts of Louisiana waters were opened to shrimpers, and since most of the waters in the spill area have closed, the turtle deaths have subsided.

Shrimpers emerged as a prime suspect in the NOAA investigation when, after a round of turtle necropsies in early May, Dr. Stacy announced that more than half the carcasses had sediment in the airways or lungs — evidence of drowning. The only plausible explanation for such a high number of drowning deaths, he said, was, as he put it, “fisheries interaction.”

Environmentalists saw the findings as confirmation of their suspicions that shrimpers, taking advantage of the fact that the Coast Guard and other inspectors were busy with the oil spill, had disabled their turtle excluder devices.

The devices are so contentious that Louisiana law has long forbidden its wildlife and fisheries agents to enforce federal regulations on the devices. Last month, Gov. Bobby Jindal vetoed legislation that would have finally lifted the ban, citing the “challenges and issues currently facing our fishermen.” By contrast, Mississippi officials strengthened turtle protections by decreasing the allowable tow time for skimmers, posting observers on boats, and sending out pamphlets on turtle resuscitation.

Officials in both states say that turtles die in shrimp season even when shrimpers follow the law, from boat strikes and other accidents. They also say there have been far fewer shrimpers working since the spill, in part because many have hired out their boats to BP. That should mean fewer, not more, turtle deaths.

But there has also been illegal activity. In Louisiana, agents have seized more than 20,000 pounds of shrimp and issued more than 350 citations to commercial fishermen working in waters closed because of the oil spill. In Mississippi in June, three skimmer boats were caught exceeding legal tow times — one just hours after the shrimper had been given a handout explaining that the maximum time had been reduced, Lieutenant Armes said.

As for the piece of shrimp that Dr. Stacy found lodged in the turtle’s throat during the necropsy, it, too, pointed to shrimpers. A turtle is normally not quick enough to catch shrimp, Dr. Stacy said. Unless, of course, it is caught in a net with them.

Diagnosing Difficulties

In the necropsy lab in Gainesville, Dr. Stacy was slitting open the turtle’s delicate windpipe, looking for traces of sediment, a tell-tale sign of drowning. He finds none there, so he examines a crinkled papery membrane barely recognizable as lungs. Nothing.

“Drowning can be a difficult diagnosis,” he said. He has requested data that will show the level of commercial fishing in the area. But, he cautioned, “A lot of times our evidence is fairly indirect.”

In a sense, the necropsies so far have posed more questions than answers, demonstrating how oil has become just another variable in an already complex ecosystem. Late in June, a dolphin examined at the Institute for Marine Mammal Studies in Gulfport showed signs of emaciation, but its belly was full of fish, suggesting that it may have gorged itself after a period of difficulty finding food.

Another dolphin, its ribs broken, was hit by a boat, a catastrophe that dolphins are normally nimble enough to avoid. The veterinarian, Dr. Connie Chevis, found a tarlike substance in the dolphin’s throat. The substance will be analyzed to see if it is oil, but one theory is that the animal could have been disoriented by oil exposure, which can have a narcotic effect, rendering it incapable of avoiding a boat strike. Ms. Deangelis said the dolphins on her recent tours have been “acting like they’ve had three martinis.”

The results raise questions about oil’s indirect effects. Is crude, for example, responsible for what anecdotal reports say is a steep increase in turtles in Mississippi and Louisiana waters? The population of Kemp’s ridleys has been rebounding thanks to years of protective measures. But some scientists have speculated that the spill is driving wildlife toward the coast, crowding areas where there is more boat traffic and setting the stage for fatal accidents.

In a normal year, one or two turtles might get snagged on the hooks of recreational fishermen at the piers. Now, the marine mammal institute in Gulfport is caring for 30 such turtles, a possible indication that they are desperate for food. In recent weeks, Dr. Chevis said, she has begun to see elevated white blood cell counts and signs of pneumonia in rescued turtles, both of which are symptoms of oil exposure, but could easily have other explanations.

In Gainesville, Dr. Stacy returned the jumbled remains of the turtle that ate the shrimp to its plastic wrapper and sent it back to the freezer. There, it will be stored indefinitely, just one piece of evidence among thousands.