Federal agents at a contentious wild horse roundup on the California-Nevada line are investigating the death of a young mustang that may have been shot before the government this week began gathering 2,000 animals from herds it says are causing ecological damage to public rangeland.

The Bureau of Land Management is "actively investigating," but it hasn't determined the cause of death of the foal that a wildlife biologist from a horse protection group found Wednesday near the roundup, BLM officials said Friday.

"BLM rangers did an initial site inspection and observed the animal appeared to have been dead for some time, preceding the start of the gather," said Jan Bedrosian, BLM deputy state director for California. "BLM special agents are actively pursuing the case as to the cause of death."

The BLM plans to round up 2,000 horses over the next month because it believes the range cannot sustain the overpopulated herds in the Twin Peaks Horse Management Area about 120 miles northwest of Reno.

Critics argue that the horses have more of a legal right to be there than the thousands of head of livestock grazing under BLM permits, but that argument has been largely unsuccessfully in court. On Tuesday, the 9th U.S. Circuit Court of Appeals refused to halt the roundup with an emergency stay sought by In Defense of Animals and others.

Craig Downer, a wildlife biologist working with the pro-horse Cloud Foundation, said he and a colleague came across the foal's carcass Wednesday.

The two-week old horse was shot in the belly and appeared to suffer other injuries, Downer said. He said it likely was killed within the past month.

"It looked as though the foal was abused, lassoed around the legs and dragged," added Chrystie Davis, an independent roundup observer accompanying Downer.

Downer, a fourth-generation Nevadan and longtime critic of BLM horse management, said he complied with a request from agency officials for copies of photographs he took of the carcass.

"They were not dismissive. They say they are going to give it a serious investigation," he told The Associated Press on Friday from Litchfield, Calif., near the roundup area.

Downer said he's worried the foal may have been the victim of growing tension between area ranchers and horse advocates over allocation of the precious forage and water resources on the high desert rangeland.

"I think it is related to the animosity against control, against having to be responsible and share the land," said Downer, who argues livestock also should be removed from the land if ecological damage is a concern.

The Twin Peaks Horse Management Area where the foal was found is not far from an area across the Nevada line where two men recently pleaded guilty to shooting and killing five mustangs in November.

Todd Davis, 45, admitted in federal court in Reno in June that he and Joshua Keathley, 36, had been drinking and used "poor judgment" when they shot the horses with an AR-15 semi-automatic rifle the weekend after Thanksgiving.

Prosecutors said they offered no plea bargain and intend to seek the maximum penalty of one year in jail and a $100,000 fine for each man at the sentencing set for Sept. 14.

"The intentional and malicious harassment, abuse and killing of federally protected wild horses should not and will not be tolerated," said Dan Bogden, U.S. attorney for Nevada.