A federal appeals court overturned the conviction of two men accused of stealing American Indian artwork Tuesday even though the judges said it was clear the men stole the centuries-old petroglyphs.

A three-judge panel of the 9th U.S. Circuit Court of Appeals ruled that the Justice Department failed to prove that the rock art taken from national forest land had market value of more than $1,000.

A federal court jury in Reno had convicted John Ligon, 40, Reno, and Carroll Mizell, 44, Van Nuys, Calif., in June 2004 of stealing government property.

The two admitted they took three boulders with etchings of an archer and bighorn sheep from the Humboldt-Toiyabe National Forest but insisted they didn't know they were breaking the law.

Forest Service officials believe the petroglyphs are at least 1,000 years old, and an agency archaeologist testified they had an archaeological value of about $8,000.

But prosecutors didn't introduce an Arizona art gallery owner's appraisal as evidence. He had estimated the boulders could be sold for $1,500 if they were in good condition but because they were badly scarred when they were removed, their retail value was $800 to $900.

"It is clear that Ligon and Mizell stole the petroglyphs. It is equally clear that the petroglyphs had a market value," Judge William A. Fletcher wrote in the opinion issued Tuesday.

But he said that the government's decision not to introduce evidence of the petroglyphs' value within the meaning of federal law left the court with little choice but to overturn the convictions.

Lawyer David Houston, who represented Mizell, said that the appeals court "agreed with us. ... This case never should have gone to the jury."

Prosecutor Robert Don Gifford said he was reviewing the opinion and had no immediate comment.