State correctional officials said Monday that a correctional officer found 81-year-old Roger Reece Kibbe dead in his cell at Mule Creek State Prison after midnight Sunday.
The suspect in his death, his cellmate, was standing nearby.
Kibbe was initially convicted in 1991 of strangling a 17-year-old who had run away from home in Seattle.
Investigators at the time said Kibbe was also a suspect in six other killings.
In 2009, a San Joaquin County District Attorney's Office investigator used new developments to connect him to the old slayings.
Authorities say he picked up most of his victims along the 5 freeway.
The Associated Press contributed to this report.
MORE | Zodiac Killer's code sent to newspaper in 1969 finally solved