The FBI is searching for the rightful owners of thousands of artifacts and the rightful resting place for thousands of human bones after the massive, illegal stockpile was seized from the home of a scientist who worked on the first atomic bomb.

Donald C. Miller ran a museum out of his home in Waldron, Ind., of artifacts he unearthed during his trips to South America, North America, Asia and Indo-Pacific regions. Some of the artifacts he had on display included Ming vases, Native American arrowheads and human remains. The FBI said Miller collected more than 42,000 artifacts over about 60 years. He died in 2015 at the age of 91.

Miller was “a renowned scientist who helped build the first atomic bomb” and opened his home to those who wanted to come see the artifacts, the FBI said. He “mostly kept hidden hundreds of human remains,” the bureau said.

The FBI said they received a tip in 2013 that Miller was in possession of the remains, many of Native American origin, at his home. In 2014, Miller, a Christian missionary, agreed to “relinquish items he had likely acquired in violation of state and federal law and international treaties,” the FBI said. The raid “did not result in Miller’s arrest or any charges,” The Washington Post reported. However, until this week, much of what the FBI found inside and details of the case were not made public.

“He [Miller] cooperated with us throughout the course of the investigation,” FBI Special Agent Tim Carpenter said, “and it was his wish that we take these objects and return them to their rightful owners, and for the Native American ancestors to be reburied appropriately.”

In 2014, the FBI was able to seize 7,000 items. Carpenter called the recovery operation “complex.”

“We are not treating this material as simply evidence,” he said. “These objects are historically, culturally, and spiritually important, and you have to take that into consideration.”

The FBI noted Miller did not keep records of the artifacts, making it difficult for the FBI to return them to their rightful owners.

Carpenter told CBS News in an interview this week that 2,000 human bones, believed to have been taken from Native American burial sites, were found in the home and the FBI believes the remains “represent about 500 human beings.” Carpenter said Miller admitted he came by most of the artifacts illegally and revealed he went to unsanctioned archaeological digs across the globe.

The FBI said they were working with Native American tribes to return the remains back to their proper burial ground. Carpenter said returning the remains was the most important part of the mission.

"You have to treat these people with dignity. These are human beings and people. It matters. It has meaning to people today, it has meaning to our children and their children," Carpenter told CBS News.

As for why Miller would have that many bones, Carpenter did not have an answer for that.

"I don't know. I truly don't know," Carpenter said.