Dozens of wild burros have been killed since May, and the Bureau of Land Management is offering a $10,000 reward

More Than 40 Wild Burros Have Been Found Shot, Killed in California Desert: 'This Is A Mass Killing'

Federal authorities are looking for the culprits responsible for the deaths of more than 40 wild burros in California.

The Bureau of Land Management said the burros, which are from the Clark Mountain Herd Area in San Bernardino County, were shot and discovered dead in the Mojave Desert between Halloran Springs, Calif., and Primm, Nevada.

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The animals’ carcasses have been found in various stages of decomposition since May.

“This is a mass killing of these animals,” Grace Kuhn with the American Wild Horse Campaign tells PEOPLE. “They were shot and left for dead. They are America’s treasure and they should be treated as such — instead they are being treated as trash.”

In one case, Kuhn says an officer found a group of 16 dead.

There are only 16,200 burros living in the western United States. There are 4,700 in California.

Kuhn says the 42 burros that were killed came from a herd of 120.

“They were all healthy,” she says.

The animals have been federally protected since 1971. Wild horses are also federally protected, as is the bald eagle.

Kuhn says the vast majority of the herd was moved by the park service in the 2000s.

“It seems like the burros were scapegoated because of cattle impacts,” she says. “Before the Wild Horse Act in the 1950s, both burros and mustangs were hunted by ranchers and the hostility towards these animals continues until this day.”

Burros, Kuhn says, are considered by some people as scourges and invasive species.

“I believe it is pure hostility to eliminate the animals from the area,” she says. “They aren’t seen as the cultural icons they should be.”

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Kuhn suspects that the person responsible is most likely a hunter or a property owner annoyed by burros foraging on their property.

“A regular Jim or Jane that has hostility towards burros,” she says. “It really can be anybody. The average American or someone influenced by commercial interests.”

Killing them is not the answer, she says.

“If there is an issue with a burro in the area, there is a way to mitigate the problem which doesn’t result in killing the animal,” she says. “You don’t have to brutally gun them down.”

The Bureau of Land Management has offered a $10,000 reward leading to an arrest and conviction in the case.