Authorities are calling it one of the largest animal massacres ever recorded.

Since May, 42 wild burros, a type of donkey, have been shot dead in the Mojave Desert. Burros are federally protected and considered a cherished symbol of the American West.

Officials with the US Bureau of Land Management in California say anyone found guilty of these killings could face 42 years in federal prison — one year for every slain donkey. They are also offering $50,000 for information leading to their killer. There are 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, told the LA Times.

He added that burros are “iconic” and “part of our national heritage.”

According to their spokeswoman Sara Webster, the Clark Mountain Herd Area was home to about 120 wild burros — that is, before a third of their group was slain. Webster believes the gunman was firing a rifle from a distance and aiming for the animals’ necks.

This isn’t the first time the West’s burro population, originally brought from North Africa by early Spanish colonists, has been under threat. In the 1950s, the LA Times reported a mass killing of burros so widespread that the Society for the Prevention of Cruelty to Animals stepped in to save them. It eventually got Congress to pass the Wild Free-Roaming Horses and Burros Act of 1971.

It’s estimated that some 16,000 wild burros live on federal “herd management areas” across five states, and officials have made many attempts to curb overpopulation — but shooting is not one of them.