At first, Richard Johnston thought nothing of an online auction house's offer to sell a letter written in 1900 by a member of Butch Cassidy's infamous Wild Bunch for $5,999. It was only later that the Old West history buff from Reno made a surprising discovery: The two-page letter from outlaw Willard E. Christiansen to Utah Gov. Heber Wells was stolen from the Utah State Archives.

"I checked my files and discovered that I had seen the letter there in 1976," Johnston recalled. "As a historian, it makes you angry that these types of thefts are occurring."

Unlike countless cases at other institutions, this story had a happy ending: Utah officials confirmed the theft after Johnston contacted them last year, and the letter was returned to the archives.

The letter, concerning Christiansen's promise to chase outlaws as a condition of his release from prison by Wells, is only one example of a historic document turning up stolen. It is part of what some say is a national problem.

"Documents used to be the least sexy thing among cultural objects for thieves to steal, but not anymore," Nevada State Archivist Guy Rocha said.

While the scope of the problem is difficult to assess due to a lack of government statistics, many historians, librarians and dealers think the thefts are on the rise because of the soaring value of such rare documents.

The popular PBS program "Antiques Roadshow" has made the public aware of the value of historical treasures, they say, and eBay and other online auction sites are making it easier to sell stolen documents.

"I'd be surprised if most states haven't been hit by theft," said archival consultant Mimi Bowling, who conducts security workshops for the Chicago-based Society of American Archivists. "I think we've always had a problem, but I think because of the rising values that the problem is on the increase."

That opinion is not unanimous. Everett Wilkie Jr., chairman of an American Library Association committee that reports theft cases brought to its attention, questions the severity of the problem.

"If I were going to guess, I don't think the problem is any worse," said Wilkie. "I think it's just being reported more, and people are more aware of it."

But Wilkie and other experts agree that a spike in prices has fueled a string of document thefts from historical societies, libraries, archives and other governmental agencies across the country.

Among them:

• In December, four men were sentenced to seven years in prison for the 2004 theft of $735,000 worth of rare manuscripts and sketches, including drawings by naturalist John James Audubon, from the Transylvania University Library in Lexington, Ky. The four were accused of tying up and stunning the librarian with a Taser in what Wilkie called the first armed robbery targeting historic documents in U.S. history.

• Last May, Howard Harner Jr. of Staunton, Va., was sentenced to two years in prison for smuggling more than 100 documents out of a National Archives research room from 1996 to 2002 and selling them through auctions. Among the stolen documents were ones signed by Confederate President Jefferson Davis and Gens. Philip Sheridan and George Custer.

• In 2002, Shawn Aubitz was sentenced to 21 months in prison for stealing hundreds of historic documents from 1996 to 1999 while curator at the National Archives branch in Philadelphia. Among the items taken were presidential pardons and autographed photographs of Apollo astronauts.

The thefts by Aubitz and Harner were uncovered when history buffs noticed some of their documents posted for sale on eBay.

The stolen letter by the Wild Bunch member was found on a Las Vegas-based auction house's Web site.

"In some ways, technology is a blessing in disguise," Wilkie said. "Unfortunately, it can also work against us."

Bowling attributes the thefts to lax security and foxes like Aubitz guarding the hen house. At her security workshops, Bowling urges institutions to do a better job of screening employees, noting more than half of all thefts may be committed by people on the payroll.

"Insider theft can be very easy," said Bowling, former curator of manuscripts at The New York Public Library. "Institutions should exercise as much care in the hiring of staff as possible."

Utah officials acknowledge the person who stole the letter written by the Wild Bunch member probably will never be caught, since the theft occurred in the early 1980s and the Las Vegas firm bought it from a company that went out of business.

The man who found it is just glad to see the letter returned because of its historic worth.

"It is a crime that material is being stolen from so many archives for it deprives future researchers insight into the past," Johnston said.