Officials are investigating a homicide after an incarcerated man, who was received from Riverside County, died at a San Quentin prison Tuesday afternoon, Sept. 17. Prison staff responded to an alarm at 3:30 p.m. and went to a cell at the San Quentin Rehabilitation Center, where they found 70-year-old Mark Squires unresponsive and called for medical assistance and first responders, according to a news release from the California Department of Corrections and Rehabilitation. Staff started life-saving measures and continued treatment at the prison’s triage and treatment center. Medical staff from an outside facility pronounced Squires dead at 4:04 p.m. Officials are investigating his death as a homicide. The suspect, Squires’ cellmate, Gustavo Lopez, was placed in restricted housing pending investigation by the prison and the Marin County District Attorney’s Office. The county coroner will determine Squires’ cause of death. Squires was received by the prison system in January 2000 from Riverside County. He was sentenced to life with the possibility of parole for lewd or lascivious acts with a child under 14 with force or violence and failure to register a specific sex offense, the CDCR news release said. Lopez, 36, arrived at the prison from Riverside County in January 2020 to serve 13 years and four months for second-strike offenses for kidnapping, corporal injury resulting in traumatic condition and false imprisonment with violence, the release said. Related Articles Crime and Public Safety | Woman’s body found in Pomona, homicide investigation launched Crime and Public Safety | Man suspected of killing sister in Riverside is arrested Crime and Public Safety | Inmate is stabbed to death at Banning jail Crime and Public Safety | Woman shot to death in Riverside Crime and Public Safety | Neighbor accused of murdering couple at Redlands nudist resort pleads not guilty