Sometime in the mid-1980s, someone made their way into the Biblioteca Nazionale Marciana in Venice, Italy, and walked out with a 500-year-old Latin copy of the first letter Christopher Columbus wrote to Ferdinand, King of Spain, describing the wonders he beheld in the Americas.

This week, after a lengthy investigation, U.S. officials announced in Delaware that the letter, which bears an estimated market value of $1.3 million, had been recovered and authenticated, and would eventually be returned to Venice.

“Culturally significant artifacts are assigned a monetary value in the world’s marketplaces in which they are traded,” said William Walker, acting special agent in charge of Homeland Security Investigations, Philadelphia. “But the cultural and symbolic worth of these objects far surpasses any given dollar value to the nations to whom they rightfully belong."

The U.S. Attorney’s Office in Delaware, ICE, and the Italian Carabinieri Command for Protection of Cultural Heritage worked together on the probe. They would not provide details of how the letter entered the U.S. or where it had been seized.

Experienced art investigators in Delaware, who have tracked down three other stolen Columbus letters in recent years, served as key investigators, a U.S. spokesperson said.

According to court papers, the Venice library acquired the letter, known as a Plannck I edition, in 1875. Sometime between 1985 and 1988, the letter was stolen. That was apparently that.

But in May 2003, court records say, a collector unknowingly purchased the letter from a rare-book dealer in the United States. Then, last year, after lengthy investigation, authorities contacted the owner of the letter, who agreed to turn it over for examination.

Paul Needham, librarian at Princeton University’s Scheide Library, was brought in to inspect the document. Needham had no doubt that the letter he was examining was the same letter gone missing from Venice nearly 40 years ago.

The individual in possession of the letter agreed to relinquish title.

"I'm just very happy that a national treasure of Italy is going back to a great library in Italy," Needham told CNN. "The American owner really did the right thing. It must be very disappointing to learn that years later, a book you bought that you paid a lot of money for turns out to be stolen."

U.S. District Court in Delaware has ordered the letter be returned to the Biblioteca Nazionale Marciana.