Delbert Miller, 69, was found unresponsive in his cell Thursday morning and was pronounced dead a short time later. Investigators identified Miller's cellmate, 20-year-old Kyle Osborn, as the suspect.
Osborn has been moved to a segregation unit.
Miller had been sentenced to 81-years-to-life for a murder in Humboldt County in 2008. He had also had a prior conviction regarding the rape of a minor.
Osborn had been convicted of sex acts with a child under 14, as well as burglary and robbery charges in 2009.