Detectives say officers found Antonio Vasquez, 27, unresponsive, covered by a blanket during a security check around 10 p.m. on Saturday.
Emergency crews tried to revive him, but he was pronounced dead a short time later.
Vasquez was serving a 22-year sentence for second-degree attempted murder.
His cellmate, Adrian Madrigal, 31, is a suspect in the homicide. He was sentenced to more than 33 years in prison for voluntary manslaughter.
Madrigal has been moved to segregated housing while the investigation continues.