A long-retired National Parks Service official has been charged with stealing ancient Native American remains from a museum collection at a sacred tribal burial site in Iowa and keeping them for more than 20 years.

Former Effigy Mounds National Monument superintendent Thomas Munson was charged Tuesday with embezzlement of government property after a lengthy investigation that tribes and archaeologists have followed closely. He's expected to be arraigned Wednesday at the federal courthouse in Cedar Rapids.

Munson, 76, has been under investigation since 2011, when he returned a box of prehistoric bones to a museum on the grounds of the 2,500-acre park in northeast Iowa. The box contained fragments of skeletons, such as teeth, jaws and leg bones, that are believed to be 500 to 2,000 years old and that were discovered there in the 1950s. Investigators later found additional boxes of remains at his home.

Munson has said he took the bones and stored them in the garage of his Prairie du Chien, Wisconsin home. His motivation isn't clear, but some tribes have said they believe Munson was trying to circumvent a law that took effect later in 1990 that required museums to return some remains and burial objects to tribes. Some archaeologists—Munson isn't one—believed the law would harm research.

The revelation outraged the 12 tribes affiliated with the site, who were already angry at monument officials for illegally building boardwalks and other structures throughout it. Located in the wooded hills along the Mississippi River, the park features 200 Native American burial and ceremonial mounds, some of which are shaped like animals.

Neither Munson nor his attorney, Leon Spies, immediately responded to voice messages seeking comment. Spies has said that Munson, who served as superintendent from 1971 to 1994, was a dedicated federal employee and is cooperating with investigators.

Iowa's state archaeologist, John Doershuk, said the criminal charge filed against Munson was important and overdue because it wasn't just ancient relics that were taken, but the remains of more than a dozen actual people.

"These are people and there are living peoples who care deeply about these remains, just as most modern Americans would about their ancestors," he said.

Doershuk said the remains "were treated very disrespectfully" when they were hastily removed. They were jumbled together and then poorly stored and have degraded somewhat as a result, he said. The sealed remains are being held as evidence, but they are expected to be returned to tribes for reburial once the case concludes.

Another former monument employee, Sharon Greener, has said that Munson directed her to pack up the bones in cardboard boxes when she was a part-time ranger in 1990 and that Munson drove off with them. She said she kept an inventory of what was taken and that when she later learned the removal wasn't handled properly, she told future superintendents what had happened.

Munson denied having the bones over the years. But that changed in 2011 after Greener brought the missing remains to the attention of a new superintendent, Jim Nepstad, who launched an inquiry. Soon, Munson returned the first box.

The National Park Service later accused Greener of a "lack of candor" for her role in the removal, eventually firing her in 2013. But in a settlement last year, the agency reinstated Greener with back pay, then allowed her to retire early.

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