The U.S. has agreed to return the remains of 18 dinosaurs, which were smuggled into the country, back to Mongolia, officials announced Thursday.

"This is a historic moment for the U.S. Attorney's office, in addition to being a prehistoric event," said Manhattan U.S. Attorney Preet Bharara, who displayed some of the bones during a repatriation ceremony in Manhattan, according to Reuters. "A recovery of this sort is really without precedent."

The remains include two Tyrannosaurus bataar skeletons and a fossilized egg, along with rock matrix containing at least four Oviraptors.

James T. Hayes, special agent in charge of the New York office of Homeland Security Investigations, said that the U.S will eventually return about 31 dinosaur remains. Investigators found that the skeletons were shipped out of Mongolia between 2005 and 2012, Associated Press reported.

"The fossils returned today do not belong in the hands of any private collection or one owner. They belong to the people of Mongolia where they will be displayed in their national museum alongside the Bataar ICE repatriated last year. HSI will not allow the illicit greed of some to trump the cultural history of an entire nation," said Hayes, in a news release.

According to the Investigators, Eric Prokopi - a self-described "commercial paleontologist" - had smuggled the dinosaur bones into the U.S.

Prokopi was arrested October 17, 2012, on "one count of conspiracy to smuggle illegal goods, possess stolen property, and make false statements, one count of smuggling goods into the United States, and one count of interstate sale and receipt of stolen goods," a press statement read.

Prokopi pled guilty to the charges on December 27, 2012. His cooperation during the investigation led to the discovery of several other stolen fossilized remains. He has been sentenced to three months in prison, Associated Press reported.

According to Bharara, the skeletons will be displayed at new natural history museum in Mongolia.