Mark Squires, a 70-year-old inmate at San Quentin Rehabilitation Center, was found unresponsive in his cell on Tuesday, leading to a homicide investigation. Despite immediate medical assistance, Squires was pronounced dead at 4:04 p.m. His cellmate, Gustavo Lopez, 36, has been placed in restricted housing as authorities investigate the circumstances surrounding Squires' death. Squires was serving a life sentence for lewd acts with a child under 14, while Lopez is incarcerated for kidnapping and related offenses. The Marin County Coroner's Office will determine the official cause of death, and the investigation is being conducted by the San Quentin Investigative Services Unit and the district attorney's office. This incident follows another homicide at San Quentin earlier this year, raising concerns about safety in the facility.