Federal officials are investigating after five wild burros were found shot and killed in Death Valley National Park earlier this week.

In a Thursday news release, park officials made clear it is illegal to fire a gun in a national park, and said the animals' abandoned carcasses "also endanger native wildlife who inadvertently ingest toxic lead shot when feeding on the dead animals."

Burros are a close relative of donkeys derived from the African wild ass, according to the National Park Service. The equines are not native to North America and are considered an invasive species. Their populations grow approximately 20% per year, compete for scarce resources with native wildlife like bighorn sheep and can damage the native vegetation, the service says.

While public land managers have been working to remove the animals from the "sensitive desert environment," the news release says the five burros killed were not part of any internal removal efforts. "Park officials say these irresponsible actions are not warranted," the release goes on.

Officials asked anyone with information about the killings to contact the National Park Service tip line.

Burros were originally brought to North America by Christopher Columbus in 1495, according to the National Park Service. During the Gold Rush, Mexican explorers brought the animals to California. Many were later abandoned or escaped and are the ancestors of today's invasive population.

In 2022, the National Park Service estimated there were more than 4,000 burros in Death Valley. The service worked with the Bureau of Land Management last year to gather some of the burros and put them up for adoption or sale.

In 2019, there was a spate of burro killings in the Mojave Desert, with 42 burros found illegally shot to death near the California-Nevada border over several months. The Mojave Desert burros, unlike those in Death Valley, are protected under the Wild Free-Roaming Horses and Burros Act and therefore can't be killed, harassed, captured or branded.