A California serial killer who authorities say was known as the “I-5 Strangler” was found choked to death and unresponsive Sunday in his cell at Mule Creek State Prison southeast of Sacramento.

Police say Roger Reece Kibbe, 81, strangled and raped at least seven women.

An autopsy showed Kibbe had been manually strangled, the Amador County Sheriff’s Office said, calling the death a homicide.

No charges have been filed in the death of Kibbe.

He was initially convicted in 1991 of strangling Darcine Frackenpohl, a 17-year-old who had run away from her home in Seattle. Her nearly nude body was found west of South Lake Tahoe below Echo Summit in September 1987.

Investigators said then that they suspected him in other similar slayings.