The stolen copy of a letter written by Christopher Columbus in 1493 has been returned to the Vatican on Thursday following a joint investigation by U.S. Homeland Security agents and Holy See antiquity experts.

The letter, which has been valued at $1.2 million, was presented to the Vatican by ambassador Callista Gingrich during a handover ceremony at the Vatican Library.

'It is a precious piece of history and I am honored to return it to its rightful owner,' Gingrich said, as reported by Catholic Herald.

The document was one of several copies of the original letter written by Columbus in Spanish detailing his discoveries to King Ferdinand and Queen Isabella of Spain. The explorer's letter was reprinted in Latin to spread the news of his find to the papacy and other royal courts across Europe.

A copy of the Latin letter was supposed to be kept at the Vatican Library, but an expert studied the document and suspected that it was fake due to a discrepancy in the stitch marks on the pages.

The authentic copy had been purchased by the late American collector David Parsons for $875,000 in 2004, according to Reuters.

According to Catholic Herald, the Latin letter, printed in Rome in 1493, was donated to the superior general of the Jesuits in the 1800s. It became a part of the Vatican Library collection in 1921 when it was presented to Pope Benedict XV.

In 2011, the expert evaluated the letter in the U.S. and concluded that it was the real letter. It was later found that the marks in the U.S. copy matched with the binding marks on the leather cover of the letter still being kept at the Vatican.

The unnamed expert notified Homeland Security art investigators about the discrepancy, prompting the joint investigation with Vatican experts.

The investigators concluded that the original document had been replaced by a forgery some time after it was acquired by the Vatican.

'We do not know exactly when the substitution took place. We will probably never know who the forger was,' said Archbishop Jean-Louis Brugues, the Vatican's chief archivist and librarian, according to Reuters.

It was found that the authentic letter had been sold to a New York book dealer by Marino Massimo De Caro, who is currently incarcerated in Italy for stealing 4,000 ancient books and manuscripts from libraries and private collections.

After the ownership of the authentic copy was traced to Parsons, his widow agreed to return the letter to the Vatican.

Msgr. Cesare Pasini, the library's prefect, said that the Vatican will keep both the forgery and the authentic letter 'because even the theft is part of history.'