Federal authorities are investigating the vandalism of a series of petroglyphs believed to be thousands of years old.

The rock panels were damaged last month by gunfire — as many as 35 shots destroyed the ancient art located southwest of Fillmore, the Bureau of Land Management reported.

"It's a shame that people do this," Sherry Hirst, field manager for the BLM's Fillmore office, said Friday.

The petroglyph vandalism was discovered earlier this week by a group of contractors taking inventory of archaeological sites in the area. The rock art is believed to be thousands of years old and had been untouched until May 2007, the BLM said.

The area is popular, BLM archaeologist Misti Haines said Friday. "These are things the Boy Scouts like to come out and visit. We have some very passionate volunteer groups, they come out to our area almost every year. They help us record these sites," she said.

The damage is irreparable, Haines said. However, the BLM will take steps to make the damage less apparent. A professional rock art conservator will be "inpainting" the damaged panels.

"It's a very specialized skill," Haines said. "They take a special palette of colors out, they go to the art and fill in visually the missing gaps."

The BLM is offering a reward for information leading to the arrest and conviction of the vandals. Anyone with information is asked to call 1-800-722-3998 or 435-743-3100.

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E-mail: bwinslow@desnews.com