Scientists regularly visit the rocky deserts of Death Valley National Park to record and study fossils that dot the Park’s terrain. During a recent trip, researchers noticed that several fossilized footprints, which had been left in a lakebed by mammals and birds, were missing. Now, Ryan F. Mandelbaum reports for Gizmodo, the National Park Service is asking for the public’s help in identifying a group of backpackers who may have information on the ancient tracks.

In a statement, the National Park Service released the photos of three men who might have witnessed the crime or have knowledge about the disappearance of the footprints. Investigators are offering an award of up to $1000 “for information leading to the identification, arrest, and conviction of those responsible,” the statement reads. According to Mandelbaum, investigators are planning to interview visitors who frequented the Park at the time of theft, in the hopes that someone will be able to provide valuable clues.

Death Valley National Park, which straddles across the arid deserts of California and Nevada, is speckled with the fossilized footprints of ancient animals. The stolen tracks were formed between 3 and 5 million years ago, when critters trod across an area that used to be a muddy lakeshore, Michael Edison Hayden reports for ABC News.

The theft of these prints is only the most recent in a string of destructive incidents to take place in Death Valley, Henry Brean notes in the Las Vegas Review-Journal. In April 2016, for instance, a group of men threatened the habitat of an endangered fish when they went on a booze-fuelled trip through Devil’s Hole. In August, someone took a wild drive through a dry lake known as Racetrack Playa (the name is by no means an invitation to joyride), scarring its delicate grounds.

Destroying—or pilfering—the property of national parks is prohibited by law. “It’s illegal to collect fossils, rocks, or anything else in National Parks,” Park Superintendent Mike Reynolds said in a statement about the fossil thefts. “The purpose of National Parks is to conserve the landscape and everything it contains for the next generation. I ask that visitors come and enjoy all there is to see, and to leave it unimpaired for others to enjoy.”

The National Park Service has asked anyone with information about the stolen fossils to call the Investigative Services Branch at 1-888-653-0009.