(CNN) -- A copy of a letter written by Christopher Columbus in 1493 that was stolen from the Vatican Library and found in the United States was returned Thursday.
Callista Gingrich, US ambassador to the Holy See, brought it back to the Vatican on Thursday following a seven-year investigation.
The letter describes Columbus' first impression of Caribbean islands to King Ferdinand and Queen Isabella of Spain. After being translated into Latin, several copies of the letter were dispersed across Europe. About 80 copies have survived, with one going to the Vatican Library in 1921.
It's unclear when it was stolen from the Vatican's collection and swapped with a forged copy, according to a statement from the US Embassy to the Holy See.
In 2011, a researcher at the Vatican Library tipped off the US Department of Homeland Security that the letter might be a forgery, the embassy said. The department contacted the Vatican, and an investigation began.
The original letter was traced to Robert Parsons, an Atlanta actuary, who had bought it from a New York rare book dealer in 2004 for $875,000, according to the embassy statement.
Parsons was unaware the letter been stolen, according to DHS. His widow, Mary Parsons, voluntarily agreed to give it back to the Vatican, said the DHS' Jamie McCall who worked on the investigation.
"She understood the significance of this particular letter to the Vatican, to the world, to the researchers that come here to the library in search of answers for what occurred during that period of time," McCall told CNN.
An investigation is ongoing, and DHS won't comment on any suspects.
Timothy Janz, director of printed books department at the Vatican Library, told CNN he has a theory about the theft, based on the intricate stitching used to bind the letter.
"It was probably done by a binder," Janz said. "Sometimes we do send books out to be bound. ...
"I doubt very much that it was a researcher who was reading. In the reading room, you could not possibly do this."
Janz said security at the Vatican Library was improved about 10 years ago.
"In 2007, there was hardly any security in this library, even stacks were basically open doors," he said. "The security was that readers didn't know that!"
The library has updated security measures since then, and Janz said he is confident such a theft could not happen today.
The letter marked the third recovered copy to be returned to European libraries by DHS.
Last week, a copy was returned to the Library of Catalonia in Spain, and another copy went to the Riccardiana Library in Italy in 2016.
CNN's Hada Messia contributed to this report.
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