The letter had been stolen from Florence's Riccardiana library and replaced with a forgery that no one noticed until Italian authorities in 2012 launched a broad investigation across Italy into plundered rare books following some high-profile thefts.
The original, meanwhile, had been sold to a rare book collector in Switzerland in 1990, then purchased by another collector at a Christie's auction in 1992 in New York. It was finally bequeathed to the Library of Congress in 2004 by the estate of its final owner, Italian and U.S. officials said Wednesday.
"Five hundred years later, it did the same trip (as Columbus), round-trip," Italian Culture Minister Dario Franceschini told a news conference in Rome with the U.S. ambassador by his side to announce the letter's return.
U.S. Ambassador John Phillips declined to identify the estate that gave the letter to the Library of Congress, saying the investigation was still ongoing. But U.S. officials said both the final owner and the Library acquired the letter in good faith, assuming its provenance was legitimate.
The auction price was 400,000 euros, but Italy's Carabinieri art squad estimates its true value at 1 million euros ($1.13 million).
The head of the Riccardiana library, Fulvio Stacchetti, said the letter was likely substituted with a fake in 1950-51, when the Riccardiana loaned the letter to national library authorities in Rome. He said that was the only time the document had left the Riccardiana, and that it would have been impossible for it to have been substituted with a fake while it was home because the reading room is so closely monitored.
The letter is one of about 30 authentic, reprinted copies of Columbus' original letter, which was handwritten in Spanish in April 1493 and almost immediately reprinted in Latin by the Rome printer Stephan Plannck.
Columbus had sent the letter to Spain's King Ferdinand and Queen Isabella to offer his first impressions of the riches that were to be found in what he believed to be the eastern edge of Asia.
"I discovered many islands inhabited by numerous people," Columbus wrote, according to a translation of the letter by the Gilder Lehrman Institute of American History. "I took possession of all of them for our most fortunate King by making public proclamation and unfurling his standard, no one making any resistance."
He described the fertility of the lands, the variety of fruit trees that he said looked "as green as lovely as they are wont to be in the month of May in Spain."
"The convenience of the harbors in this island and the excellence of the rivers, in volume and salubrity, surpass human belief," he wrote.
Columbus noted that the native people he met had no weapons and were "fearful and timid ... guileless and honest" — and easily conquered by Spain. In the letter, he wrote that he had given them gifts to win their affections in hopes "that they might become Christian and inclined to love our king and queen and princes and all the people of Spain."
The investigation began in 2012 after Italy's national library in Rome reported the theft of some important books from its collection. It was the same year that Naples' famed Girolamini library reported that thousands of precious books had been looted from its collections, allegedly by its director.
During the course of the investigations, experts ascertained that the national library's copy of the 1493 Columbus letter was a fake. As the probe expanded, they determined that the Florence version was also a modern-day forgery.
Gen. Mariano Mossa of the Carabinieri's art squad police said the two forgeries bore the same incompatibilities with the original: different typefaces and paper sizes, different pens and pencils used to number the pages, and the absence of the Florentine library stamp.
"The result was that it looked photocopied," he said.
Contacts were immediately started with officials from the Homeland Security office, since the U.S. is a known destination stolen rare books, he said.