A criminal complaint was unsealed in federal court in Detroit against Vincent Munster, who is chief of the virus ecology section at Rocky Mountain Laboratories in Hamilton, Montana, and Claude Kwe, who works with him.

Munster and Kwe were stopped at Detroit Metropolitan Airport in January after a flight from Paris and nine days in the Republic of Congo. An outbreak of the mpox disease has been linked to more than 2,000 deaths in Congo, a vast region in central Africa, though a two-year outbreak was declared over in April.

Munster “adamantly denied” returning to the U.S. with biological materials or samples, the FBI said in a court filing.

But tests subsequently revealed that Munster and Kwe were traveling with vials of deactivated mpox, the FBI said, yet they had failed to declare them or obtain the necessary permission.

“Any deliberate effort to conceal and smuggle biological materials into the United States without proper authorization is a breach of the public’s trust and could have placed the public at risk,” said Marcus Sykes of the Office of Inspector General at the Department of Health and Human Services.

Munster and Kwe did not immediately respond to requests for comment. They are expected to appear in federal court in Missoula, Montana, on Wednesday.

“This matter is currently under investigation, and NIH is cooperating fully with law enforcement and appropriate authorities,” said the National Institutes of Health, which oversees the lab. “Because this is an ongoing investigation and personnel matter, we are limited in what additional information we can provide at this time.”

There was no mention in the government’s court filing about why Munster and Kwe may have wanted to bring the deactivated mpox virus to their lab. But they are virologists who have worked extensively on mpox research, the FBI said.

Munster told investigators at the Detroit-area airport that any necessary documents were in his laptop, “but you don’t need them. I do this all the time,” the FBI quoted him as saying.

“It is reasonable to believe that Munster’s statements regarding the possession of the required documentation to (customs officers) were materially false,” the FBI said.

The most common symptoms of mpox, according to the World Health Organization, are a rash and fever, but it can sometimes cause serious illness. Most people recover fully.

Mpox, previously known as monkeypox, was first identified by scientists in 1958 during outbreaks of a “pox-like” disease in monkeys. Until a few years ago, most human cases were seen in people in central and West Africa who had close contact with infected animals.

In 2022, the virus was confirmed to spread via sex for the first time and triggered outbreaks in more than 70 countries that had not previously reported mpox.