VATICAN CITY — When American authorities contacted officials at the Vatican Apostolic Library in 2012 to tell them that a prized copy of a letter by Christopher Columbus in its collection could be a fake, the Vatican’s librarians almost didn’t believe it.

“I must admit it took us rather by surprise,” the Vatican’s archivist and librarian, Archbishop Jean-Louis Bruguès, said Thursday, during a ceremony to celebrate the return, six years later, of the original document. It had been stolen, replaced with a skillfully crafted forgery and sold in 2004 to an actuary from Atlanta.

But the return of the document, which describes Columbus’s first impression of Caribbean islands, points to a tantalizing mystery with no solution in sight. This is the third time in two years that American officials have returned to three European libraries stolen copies of the Columbus letter that had been replaced with a forgery. Investigators are still trying to determine who committed the thefts and if there are links between the three cases.

And while dramatically increased security at the Vatican may go a long way toward preventing a recurrence here, the case is a window into the shadowy world of antiquarian forgeries.