A repeat felon from Riverside County serving a 20-year sentence at the state prison in Lancaster was allegedly killed by his cell mate, the California Department of Corrections & Rehabilitation reported this week.

Robert Allen Garten Jr., 50, was fatally injured Nov. 9 at California State Prison in Los Angeles County, according to CDCR Lt. Richard Ochoa.

He said guards conducting block checks about 8:45 p.m. found Garten “unresponsive in his cell” and immediately alerted medical staff, who had the unconscious, gravely injured inmate taken to a hospital, where he died less than an hour later.

Garten’s cell mate, 32-year-old Stephen Anthony Marrufo, also from the Riverside metropolitan area, was identified as a suspect and placed in a segregation unit while an investigation is conducted by Los Angeles County sheriff’s deputies, according to Ochoa.

Garten’s remains are awaiting autopsy.

Ochoa said Marrufo had bruises on a foot and one of his hands, as well as red blotches on the back of his neck.

The convict is serving a life-with-parole sentence for first-degree murder. He was incarcerated on Nov. 6, 2015.

Garten was incarcerated on April 21 of that year. According to court records, he pleaded guilty to solicitation to commit murder and assault with a deadly weapon.

Records show that Marrufo pleaded guilty to the murder count in October 2015. He had served time in prison before that conviction.

A possible motive for the attack was not disclosed.