A precious copy of a letter written by Christopher Columbus in 1493 regarding his discovery of the New World, which had been stolen from a library in Florence, has now been returned to Italy by the US after it was found in the Library of Congress in Washington, DC.

The rare eight-page document valued €1m (£800,000, $1.1m) by the Ministry of Cultural Heritage and Activities and Tourism (MiBAC) was switched with a forgery at Florence's Riccardiana library.

The swap was noticed only a few years ago, triggering an international hunt that eventually led to the Library of Congress. Addressed to Spain's King Ferdinand and Queen Isabella, the letter contains Columbus' first impressions of the American continent.

"Five hundred years later, it did the same trip [as Columbus], round-trip," said Italian culture minister Dario Franceschini with US ambassador John Phillips at a conference marking the letter's return in Rome.

The circumstances of the manuscript's first voyage across the ocean remain mysterious and are being investigated by US detectives.

According to the head of the Riccardiana library, Fulvio Stacchetti, the paper was stolen in either 1950 or 1951 when it was loaned to national library authorities in Rome. Librarians realised the letter had been replaced in 2012, but Stacchetti maintained that the swap could have happened only in Rome as that was the only time the document was let out of the Riccardiana, where readings are closely supervised.

About 30 years later, the original was purchased by a rare book collector in Switzerland and resurfaced in 1992, when it was auctioned at Christie's in New York. In 2004, it was bequeathed to the Library of Congress by the estate of its final owner, whose identity was not released, but was said to have acquired the historic text in good faith.

Meanwhile, the Riccardiana stamp impressed on the original had been bleached out to disguise its provenance. The letter is not the original penned by Columbus in Spanish, but one of less than 20 prints of a Latin translation made by Stephan Plannck in 1493.