An inmate at the Yellowstone County Detention Facility was found unconscious in his cell over the weekend and died later at the hospital, according to Sheriff Mike Linder.
Linder said CPR was performed on the man and he was transported to a Billings hospital, where he died.
The 54-year-old man has not yet been identified.
In a press release Monday afternoon, Linder said the man had a pulse when he was transported from the jail but died in the hospital on Sunday. He was found unresponsive in his cell at approximately 10:20 a.m., according to the press release. Another inmate in the cell notified jail staff when he saw that the man was lying in his bed, not moving.
Linder said the man was a Department of Corrections inmate.
An autopsy was conducted on Monday that showed no obvious signs of foul play, the press release stated. Toxicology samples will be sent to the state crime lab "to determine if the man may have ingested something."
Linder said there would be no determination on cause or manner of death until the toxicology results were returned. Results can take up to three months to be returned, he noted.
"There's no evidence it was a suicide attempt or anything like that," Linder said, in an earlier phone call.
Similarly, Linder said there was no early indication the man had been assaulted.
"We're not sure — it must be medical," he said.