In 1921, the Vatican Apostolic Library acquired an extraordinary document: a copy of a letter that Christopher Columbus wrote in 1493, describing his first impressions of the Caribbean islands to the Spanish monarchy. Some 90 years later, American officials contacted the library with jarring news. The letter in the Vatican’s collection, they suspected, had been stolen and replaced with a near-perfect fake.

As Elisabetta Povoledo reports for the New York Times, the original document was ultimately located in Atlanta, Georgia, and returned to the Vatican last week. But questions continue to swirl around this strange case. Officials still do not know when the letter was stolen, or by whom. They also are unsure if the Vatican theft is connected to similar crimes that took place in two other libraries.

The eight-page letter, which has been valued at $1.2 million, is a copy of a message that Columbus penned, in Spanish, to King Ferdinand and Queen Isabella of Spain. In his letter, Columbus describes a land “full of trees of endless varieties, so high that they seem to touch the sky” and a native population who “firmly believed that I, with my ships and men, came from heaven,” according to Povoledo.

Columbus’ letter to the royal couple was translated into Latin and circulated widely; 80 of these copies are known to exist today. The Vatican’s document was printed in Rome in 1493 and, centuries later, bequeathed to the Apostolic Library by the collector Giovanni Francesco De Rossi. The letter had been bound with blank papers to make it appear thicker.

In 2011, United States Homeland Security Investigations received a tip from a rare book and manuscript expert who had seen the Vatican’s copy and suspected it was a forgery. Over the course of a years-long investigation, American officials were able to trace the original letter to a collector in Atlanta, who had purchased the document “in good faith” from a New York dealer in 2004, according to the United States Department of Justice.

The collector, Robert David Parsons, had paid a $875,000 for the letter. In 2017, an expert compared Parsons’ document to the one in the Vatican and determined that only Parsons’ was authentic; the other was a very skillfully executed fake.

But who had committed the dastardly switch, and how had they done it? Investigators are not yet certain, but Timothy Janz, director of the printed books department at the Vatican Library, tells Delia Gallagher and Madison Park of CNN that it “was probably done by a binder.”

“Sometimes we do send books out to be bound,” Janz adds. “I doubt very much that it was a researcher who was reading. In the reading room, you could not possibly do this.”

The Apostolic Library has vastly improved its security in recent years, and Janz tells CNN that he does not believe such a heist could be pulled off today. But the case remains mysterious, not least of all because two other European libraries have been affected by strikingly similar thefts. Earlier this month, American officials returned a copy of the Columbus letter to the National Library of Catalonia in Barcelona, which also found out that it was holding a convincing forgery of the original document. In 2016, officials returned yet another copy, which had been acquired by the Library of Congress, to the Riccardiana Library in Florence. There, too, the authentic document had been swapped out for a fake.

The forgery that was held in the Vatican had been created with a technique called “stereotyping,” which “reproduces the tactile effects of early printed books,” Povoledo writes. This technique was popular during the 19th and 20th centuries, so it is possible that investigators are dealing with a crime that was committed many years ago. Archbishop Jean-Louis Bruguès, the Vatican’s archivist and librarian, tells Povoledo that if this is indeed the case, “we will probably never know for sure who the forger was.”