ENCAMPMENT - Wildlife experts are starting to think a herd of elk south of Rawlins was literally run to death.

The cause of the massive elk die-off 15 miles south of Rawlins remains undetermined, but state wildlife experts are focusing on toxins or the possibility that excessive running and stress led to the paralysis that has affected 280 animals, which either died or were killed by wildlife managers.

Dr. Walt Cook of the Wyoming State Veterinary Lab in Laramie said two elk thought to be suffering early symptoms of the paralysis that affected those that died were killed by game officials on Monday and necropsies have been performed. Results of the necropsy are not expected until later this week.

One of the elk was "showing very early signs" of the mystery disease, Cook said, because although it was "still able to walk and run a little bit" the animal seemed to have difficulty moving its hindquarters.

The state vet lab pathologist will evaluate the muscle tissue from the affected animals to determine whether "it is a primary muscle lesion that causes them to go down" or if they are developing a secondary muscle lesion after they are already immobilized, Cook said.

If the results show that the muscle lesions are "primary," then researchers will lean more heavily toward the theory that the elk were "somehow stressed or ran in some way," Cook said.

Severe stress or long distance running can affect an animal's muscles, leading to the kind of paralysis noted in the animals that have died, he said. Should the necropsy show that the lesions are secondary - caused after the animal is already partially down - such a theory will move into the "unlikely" category of causes.

However, the idea that the elk suffered and died as a result of stress or long distance running "kind of fits," Cook said.

He said wildlife researchers first recognized the effects of running and stress on elk when they "unknowingly ran them too hard" while conducting various research projects. In those cases, the animals which had been run hard would collapse and die a week or so after the initial period of stress.

Many other kinds of reasons for the elk illness and death have been ruled out, including natural poisoning by nitrates, salt and metals. But, Cook said, "We still have the one common lesion that we're finding in these (dead elk) is muscle degeneration." As a result it is "becoming more and more likely" that the elk deaths are related to running or stress and not to toxins, he said.

Wildlife managers have found no evidence that people in motor vehicles chased the elk, nor is there any evidence that they were harassed by wolves or other predators. It would be "unusual for wolves to run them this far to cause this problem," Cook said. Further, "we would have expected wolves to have gotten one of them" and there is no evidence of any predation injuries or deaths.

Officials have no way to know if the animals might have been frightened and run by airplanes or helicopters flying in the area, he said.

Even so, a number of other possibilities still exist and are also being investigated.

Some types of livestock feed supplements "can cause this kind of syndrome," Cook said, but he noted that "seems relatively unlikely" because of the amount of supplement the animals would need to consume to feel such effects and the lack of evidence in the field of any actual supplements.

Not yet ruled out, but considered unlikely, are poisons such as Compound 1080, which has not been used legally for many years, and strychnine, which is not "very likely" because of the amount needed to cause such reaction in so many elk, Cook said.

Common plants that can cause such symptoms have been ruled out, but other plants remain a possible source, as does the "very unlikely" disease known as tick paralysis.

Four different water sources, including an artesian well that was "suspicious" because of its location in the center of the outbreak, and water associated with oil drilling activities, have been tested and found to be safe for animals, Cook said.

The elk in the area have moved into more remote areas, making monitoring more difficult, Cook said. Based on helicopter monitoring conducted Monday, he said, "I think the worst of it is over."

Wildlife managers have also captured four live elk from the herd affected by the die-off, and those animals are now being monitored to determine how they respond to various treatments to see if vitamin deficiencies are causing the illness and deaths.

The four elk - three cows and one young bull - were moved by helicopter from the herd area and then transported in a stock trailer to Laramie on Sunday and Monday.

The first elk deaths were reported by coyote hunters on Feb. 8.