Roger Reece Kibbe, 81, was discovered unresponsive on the floor of his cell at Mule Creek State Prison at around 12.45am on Sunday.
Kibbe's cellmate, convicted murderer Jason Budrow sporting a '666' tattoo on his forehead, was found standing next to the mortally injured man.
Kibbe was taken to the prison's medical facility, where he was pronounced dead at 1.30am, Fox 40 reported.
Amador County Undersheriff Gary Redman said Kibbe's death was 'being investigated as a homicide.'
Officials did not say how Kibbe died, or whether a weapon was used.
Budrow, 40, who is serving a life sentence on a 2011 second-degree murder conviction, has been moved to an administrative segregation unit. As of Monday afternoon, he has not been charged in connection with Kibbe's death.
Kibbe was serving multiple life sentences without the possibility of parole for raping and killing at least seven women and girls over the span of 30 years.
Kibbe, a married furniture maker whose brother was a police detective, was initially convicted in 1991 and sentenced to life in prison for strangling to death 17-year-old Darcie Frackenpoh.