IONE, Amador County (CBS SF) — Notorious serial killer Roger Reece Kibbe, known as the infamous “I-5 Strangler” who pleaded guilty to raping and murdering six victims including two East Bay women, has died in an attack at Mule Creek State Prison, authorities announced Monday.

State prison officials said that an officer conducting institution population counts discovered Kibbe lying unresponsive on the floor of his cell at approximately 12:40 a.m. Sunday. His cellmate, Jason Budrow, was standing near his body.

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The medical staff immediately responded and transported Kibbe to the institution health care facility where life-saving measures were unsuccessful and he was pronounced deceased at 1:23 a.m.

Budrow, who was sentenced on June 29, 2011 from Riverside County to serve a life without the possibility of parole for first-degree murder, was taken to the segregation unit pending the investigation.

Amador County Amador County Undersheriff Gary Redman told the Sacramento Bee that the death was “being investigated as a homicide.”

The 81-year-old was initially sentenced to 25 years-to-life on May 10, 1991, for the murder of Darcie Frackenpohl. A runaway from Seattle, Frackenpohl’s nude body was found at Echo Summit in El Dorado County.

In 2008, DNA evidence connected Kibbe to six more killings, dating back to 1977, to which he pleaded guilty.

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Roger Reece Kibbe

After a plea bargain to avoid the death penalty in 2009, the then white-haired 70-year-old said guilty six times as a San Joaquin County judge George Abdallah Jr. read each victim’s name and the charges.

He was sentenced to an additional six life sentences with special enhancements for rape and kidnapping for the murders of Lou Ellen Burleigh, Lora Heedick, Barbara Ann Scott, Stephanie Brown, Charmaine Sabrah and Katherine Kelly Quinones.

Burleigh disappeared after going to what she thought was a job interview in a Walnut Creek shopping center on Sept. 11, 1977. Her body was found dumped at Lake Berryessa in Napa County.

Scott was kidnapped on July 3, 1986, in Pittsburg. Her body was dumped at Lone Tree Gold Course in Contra Costa County.

At at time of his plea bargain, prosecutors said Kibbe kidnapped his victims and then tied them up with parachute cord and silenced them with duct tape.

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He then proceeded to cut open their clothes in irregular shapes with scissors that had belonged to his mother. Kibbe sexually assaulted his victims and then strangled them.