Inmate Found Dead at San Quentin Prison, Investigation Underway

Officials are conducting an investigation into the death of an incarcerated man at San Quentin prison in Marin County, California. The deceased, identified as 70-year-old Mark Squires, was found unresponsive in his cell on Tuesday afternoon, September 17. Prison staff responded to an alarm at approximately 3:30 p.m. and discovered Squires in a non-responsive state. Immediate medical assistance was requested, and first responders were called to the scene.

According to a news release from the California Department of Corrections and Rehabilitation (CDCR), prison staff initiated life-saving measures and continued treatment at the prison’s triage and treatment center. However, despite their efforts, Squires was pronounced dead at 4:04 p.m. by medical staff from an external facility.

Authorities are treating Squires’ death as a homicide and have identified his cellmate, Gustavo Lopez, as the primary suspect. As part of the ongoing investigation, Lopez has been placed in restricted housing, pending further inquiries by the prison and the Marin County District Attorney’s Office.

The cause of Squires’ death will be determined by the county coroner. Squires had been received by the prison system in January 2000 from Riverside County. He was serving a life sentence with the possibility of parole for lewd or lascivious acts with a child under 14 with force or violence, as well as failure to register a specific sex offense, as stated in the CDCR news release.

Lopez, 36, arrived at San Quentin prison from Riverside County in January 2020. He was serving a sentence of 13 years and four months for second-strike offenses, including kidnapping, corporal injury resulting in traumatic condition, and false imprisonment with violence.