It’s believed to be the largest massacre of its kind on public land managed by the U.S. Bureau of Land Management in California, the agency said.

The small donkeys are federally protected and fiercely beloved. Anyone found guilty of capturing, harassing or killing one could face up to a year of jail time. Anyone found guilty on 42 counts of killing them could face 42 years.

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The bureau and a group of animal protection organizations have offered a reward totaling more than $50,000 for information that helps catch and convict the killer or killers. In recent days, activists have redoubled their efforts and anonymous donors have chipped in, desperately trying to solve the crime before more burros die. As of Tuesday evening, investigators were still chasing tips, but had no suspects.

“We will pursue every lead until we’ve arrested and prosecuted those responsible for these cruel, savage deaths,” William Perry Pendley, the bureau’s deputy director for policy and programs, said in a statement. He called the animals “an iconic part of the American West, and part of our national heritage.”

The wild burros roamed the Clark Mountain Herd Area and lived among a group of about 120, agency spokeswoman Sarah Webster said. The summer’s shootings mean more than a third of that population has been wiped out.

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In many instances, the gunman appeared to be firing from a distance and aiming at the animals’ necks, Webster said. It’s unclear whether there was more than one shooter, but the weapon of choice appears to have been a rifle.

“We are deeply disturbed by this wanton crime and the senseless slaying of our nation’s wild burros,” Kitty Block, president of the Humane Society of the United States, said in a statement. “Anyone who is capable of this level of violence must be held accountable.”

Burros evolved in the deserts of North Africa and were first introduced to North America by Spanish colonists. They were pack mules for the west’s early European explorers, and by the Gold Rush they were critical for miners in need of strong and sure-footed helpers to transport supplies to and from their campsites.

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With no natural predator, they thrived in the arid Southwest.

Indeed, they did so well that they drew the crosshairs of hunters and frustrated farmers, who were sick of watching the donkeys graze on the grass they used to feed their livestock. In the 1950s, the Los Angeles Times reported, the mass killing of wild burros was so rampant that the Society for the Prevention of Cruelty to Animals lobbied for legislation to protect the animals.

Finally, Velma Johnston — now known as “Wild Horse Annie” — persuaded Congress to pass the Wild Free-Roaming Horses and Burros Act of 1971. At the time, the animals were quickly disappearing. But now, more than 45 years later, their proliferation has sparked new conflict between creatures and residents and between local and federal agencies, which have sparred over equine management.

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The Bureau of Land Management estimates there are about 16,000 wild burros roaming around government-supervised “herd management areas” across five states. Officials have tried a number of plan to keep their population at a “sustainable” level, from birth control to adoption. Shooting them, however, has been illegal for decades.

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