The Bureau of Prisons confirmed that an inmate was assaulted on Sunday at the United States Penitentiary Coleman II, a high security federal facility in Florida, and said no one else was injured.

Nassar was stabbed six times in the back and neck on Sunday afternoon, according to a source. He has what one source described as "substantial injuries," including what appears to be a collapsed lung.

He is listed in stable condition, according to two sources familiar with the situation.

The Bureau of Prisons confirmed that an inmate was assaulted on Sunday at the United States Penitentiary Coleman II, a high security federal facility in Florida, and said no one else was injured.

Officials have identified at least one suspect, a fellow inmate, and are looking at evidence including video footage to determine if anyone else was involved, according to a source.

Staff members who were guarding Nassar were working overtime, according to a source briefed on the investigation. The source said this was in part due to staffing shortages at the Bureau of Prisons.

Nassar was housed in a unit with all inmates convicted of sexual assault, according to a source familiar with the situation.

Nassar was convicted of state and federal charges for sexually assaulting members of the USA gymnastics team. He's serving his sentences, which total hundreds of years, at USP Coleman.