The Snohomish County Medical Examiner's office says the inmate, Jerry Jamison, died of a heart condition and his manner of death was natural.
The problem is, nobody's sure exactly when he died.
Jamison was serving time at the Monroe Correctional Complex for drug-related charges and eluding police. He had served 18 months of a five-year sentence when staff found him dead in his cell.
"As our officer approached, he met some fire personnel that said that he was already deceased," said Monroe police spokeswoman Debbie Willis.
Willis said officers described Jamison's body as "cold to the touch."
"To our officers it appeared to be a natural death," she said.
But now the Department of Corrections is investigating how long Jamison was dead inside his jail before officers found him.
A DOC spokesman said investigators know Jamison was seen moving late on the evening of September 19. He appeared to be sleeping during daily head counts until mid-day on September 21, when staff found him dead.
The DOC is doing a critical-incident review to figure out what happened between that time and if anyone else say Jamison alive.
"When they found him in his bunk he had mail on his chest and he was laying there," Willis said.
Jamison lived alone in his cell at the reformatory unit, where he was allowed to have a prison job. His body was found during his "weekend," when he didn't have to show up for work.
Head counts at the reformatory are done four times per day.
The investigation could take at least six weeks to complete.