A California serial killer known as the “I-5 Strangler” in the 1970s and 1980s has been killed in the prison where he was serving multiple life sentences, state correctional officials said Monday.

A correctional officer doing rounds spotted Roger Reece Kibbe, 81, unresponsive in his cell at Mule Creek State Prison southeast of Sacramento shortly after midnight Sunday, officials said. They said they are investigating his death as a homicide.

His cellmate was standing nearby, officials said. Kibbe was taken to a prison health care facility and pronounced dead less than 45 minutes later. Amador County’s chief coroner, sheriff’s Sgt. P. Weart, said he couldn’t give details on the death, citing the ongoing investigation.

Kibbe, a former suburban Sacramento furniture maker whose brother was a law enforcement officer, was initially convicted in 1991 of strangling 17-year-old Darcine Frackenpohl, who had run away from her home in Seattle.

Her nearly nude body was found by a jogger west of South Lake Tahoe below Echo Summit in September 1987, two to three weeks after she was killed. Her pink dress was discovered about 1,000 feet from the body.

Investigators at the time said Kibbe was also a suspect in six other killings believed linked to the “I-5 Strangler,” whose trademark was cutting his victims’ clothing in odd patterns. Several took place in the Sacramento and Stockton areas along Interstate 5.

Prosecutors then were unable to file charges in those cases, and he was sentenced to 25 years to life in prison for Frackenpohl’s death.

That changed in 2009, when a San Joaquin County District Attorney’s Office investigator used new developments in evidence to connect him to the old slayings.

Kibbe pleaded guilty to six new counts of murder in Amador, Contra Costa, Napa, Sacramento, San Joaquin and Stanislaus counties. His victims were Lou Ellen Burleigh, 21, in 1977 and Stephanie Brown, 19; Lora Heedrick, 20; Katherine Kelly Quinones, 25; Charmaine Sabrah, 26; and Barbara Ann Scott, 29, all in 1986.

It wasn’t until 2011 that a Napa County sheriff’s deputy found the remains of Burleigh — a piece of bone — in a dry riverbed near Lake Berryessa, after Kibbe agreed to help locate her body as part of a plea agreement. Burleigh was 21 and living in Walnut Creek when she disappeared in 1977 after going to meet Kibbe to talk about a secretarial job.

Prosecutors said they agreed to drop the possibility of the death penalty because Kibbe was unlikely to ever realistically face execution.

Corrections officials said he was serving two life terms without the possibility of parole from San Joaquin County from 2009, beyond the earlier life with parole sentence in El Dorado County for Frackenpohl’s death. Officials could not immediately explain the lack of other listed charges.

His cellmate is serving a life sentence with the possibility for parole for a first-degree murder in Riverside County.

Kibbe had been at the prison since 2013 but officials wouldn’t say how long he had been housed with his cellmate, citing the ongoing investigation.