Petroglyph panels at Agua Fria National Monument have been damaged by white paint and obscenities scrawled on nearby boulders, the Bureau of Land Management said this week.

Although the petroglyph site, about 2 miles east of Cordes Junction, was relatively accessible, the damaged panels were not, said monument manager Rem Hawes.

"It would actually take some climbing on cliffs to get to where these petroglyphs are," he said.

Authorities believe the vandals visited the site at least twice between July and November.

The bureau is offering a $2,500 reward for information leading to the arrest and conviction of those responsible for the damage.

Archaeologists say that Archaic Period hunter-gatherers who once lived in the area left the rock art about 2,000 years ago. The area later was settled by people archaeologists call Perry Mesa Tradition, about whom very little is known. The monument has about 400 recorded archaeological sites.

"It's very rare, and so it's irreplaceable," Hawes said. "So much of it hasn't been studied yet."

When sites are vandalized, the information they contain can be lost.

"It's losing our history, essentially," he said.

Authorities are becoming increasingly frustrated with damage to archaeological sites. Kaibab National Forest staff recently had to hire a crew to clean up paint at Keyhole Sink, a popular petroglyph site in northern Arizona. The cleanup cost thousands of dollars, and the vandals have not been caught.

Damaging petroglyphs or other archaeological sites is a violation of federal law that carries a $100,000 fine and up to a year in prison.

Anyone with information about the Agua Fria vandalism should call Special Agent Angela Stevens at 602-417-9316.

Hawes said he suspects that damage like this is typically done by youths who do not fully understand the historical value of the sites.

"This is a bad thing to do, and it's a felony and it's taken seriously," he said.

Reach the reporter at ron.dungan@arizonarepublic.com or 602-444-4847.