A set of ancient petroglyphs that were taken right out of a cliff face in California’s Eastern Sierra desert in October of last year have been recovered by the U.S. Bureau of Land Management(BLM), the Los Angeles Times reports. All five panels were recovered following an anonymous tip received in a letter, though the thieves behind the enormous heist have not yet been identified.

“Now, the healing can begin,” Bernadette Lovato, a BLM Field Office Manager, told the L.A. Times. “Recovery was a priority for me, and the public outrage intensified the need for them to be returned… We found all five panels by following an anonymous tip sent to us in a letter.”

The five petroglyphs, some of which were removed from a spot 15 feet up on the face of a cliff, was deemed the “the worst act of vandalism ever seen” on the 750,000 acres of land managed by the BLM. Despite the enormous risk and effort that went into the heist, the glyphs could not have fetched more than $500-$1,500 apiece on the black market. The BLM is offering a $9,000 reward for information leading to the arrest and conviction of those responsible.