U.S. authorities have agreed to send back fossils of at least 18 dinosaurs to Mongolia after discovering the stolen remains during an investigation.

The remains ranged from 68 million to 80 million years old, and were taken from a site in the Gobi desert, according to Reuters.

The news comes a month after paleontologist Eric Prokopi was sentenced to three months in prison for his involvement in a "smuggling scheme."

Prokopi pleaded guilty back in 2012 to three felony count, according to Reuters. He cooperated with officials in finding the fossils, including some that authorities didn't know of before talking to Prokopi.

At Prokopi's sentencing in June, officials said that the probe has led to other investigations into illegally imported fossils in California, New York, and Wyoming.

"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."

Mongolia is busy constructing a new museum where the fossils will most likely be displayed, Bharara added.

Two Tyrannosaurus bataar skeletons, a fossilized egg and a well-preserved 'nest' of several Oviraptor skeletons were among the stolen fossils.

Federal prosecutors and customs agents have found the remains from 31 dinosaurs, officials confirmed to Reuters.

At least one of the Tyrannosaurus bataar skeletons were sent back to Mongolia last year.

"Fossil looting is a huge problem worldwide," Mark Norell, curator-in-charge of the paleontology division of the American Museum of Natural History, said at the ceremony, according to Reuters.

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