Wyoming wildlife biologists and federal researchers have identified the chemical responsible for the death of hundreds of elk in southern Wyoming last year.

They say usinc acid, a substance found in lichen the elk ate, is responsible for the deaths of perhaps as many as 600 elk in a die-off that began in February 2004.

"It started with a couple of coyote hunters finding an elk that was alive but couldn't get up," said Terry Kreeger, supervisor of veterinary services at the Wyoming Department of Game and Fish. "Our people went out and found other elk down. They've look at you, but they couldn't move. You could pat them on the head."

Wildlife workers eventually found 327 dead elk on the prairie, Game and Fish biologist Greg Hiatt said. About 80 percent of them here cow elk.

"But we know we didn't find all of the bodies," Hiatt said. "Our best estimate would be that 500 or 600 died."

The cause of death had baffled scientists at the time. They took blood samples and ran exhaustive tests and dozens of lab experiments looking for poisons, toxins, viruses and deadly bacteria but found nothing.

A break came when a Wyoming biologist found an article from 1964 in which lichen was blamed for sickness and death in cattle and sheep.

"We went back out and collected bags and bags of the lichen, maybe 100 pounds or so," Kreeger said. "We had three captive elk in Jackson Hole and we brought them to our Laramie lab and began feeding them the lichen."

Two of those elk ate the lichen and died, one after eight days and one on the 10th day after feeding began. The third elk refused to eat the lichen and is still alive in a pasture at the lab.

"Two ate the lichen and died, one didn't eat it and was fine," Kreeger said. "That was the absolute smoking-gun proof that the lichen killed the elk."

The elk herd in that area has a range of several hundred square miles and often crosses into Colorado above Craig and Steamboat Springs. Because of the drought that lingered into the winter of 2003-2004, biologists say they believe the elk moved out of their normal winter range into an area filled with the lichen, which grows on rocks and on the ground.

Biologists say that about 300 elk in the herd of 800 didn't eat the lichen and eventually wandered back into their normal range. Hunting restrictions were put on the herd in the 2004 season and its now rebounded to its normal size.

Ten of the elk in the herd now wear radio collars. Last winter they didn't come back into the area where they found the lichen and game and fish officials say that if the animals try to come back this winter, they'll chase them out.

Usinc acid was once widely used by bodybuilders to burn fat. The U.S. Food and Drug Administration banned its use after tests showed it caused liver damage and some human deaths.

"Usinc acid was killing people long before it started killing elk," said researcher John Roach with the FDA in Maryland.