A California serial killer known as the "I-5 Strangler" has died in prison.

Roger Kibbe, 81, was found unresponsive on the floor of his cell at Mule Creek State Prison on Sunday with his cellmate standing nearby, the California Department of Corrections and Rehabilitation said on Monday.

Kibbe was transported to the prison's healthcare facility, where he was later pronounced dead. His death is being investigated as a homicide, officials said.

The CDCR did not immediately respond to PEOPLE's request for comment.

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Kibbe was serving multiple life sentences at Mule State Prison for several first-degree murder convictions, according to corrections officials.

He was first convicted of first-degree murder in 1991 for the 1987 killing of 17-year-old Darcine Frackenpohl, for which he was sentenced to 25 years to life in prison, according to the Sacramento Bee.

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DNA evidence in the early 2000s linked Kibbe to six other killings, the Sac Bee reports. His other victims were Lou Ellen Burleigh in 1977 and Barbara Ann Scott, Stephanie Brown, Charmaine Sabrah, Katherine Kelly Quinones and Lora Heedrick — all in 1986.

In 2009, Kibbe plead guilty to several new murder counts with special enhancement for rape and kidnapping. He was sentenced to consecutive life sentences for the plea, which allowed him to avoid the death penalty, according to the newspaper.

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