San Quentin Rehabilitation Center appears on Oct. 15, 2023. (Wendy Medina/Bay City News)

An incarcerated person at San Quentin Rehabilitation Center died Tuesday in a case authorities are investigating as a homicide, according to the California Department of Corrections and Rehabilitation.

Mark Squires, a 70-year-old man, was found unresponsive at about 3:30 p.m. Tuesday in the cell he was occupying with 36-year-old Gustavo Lopez.

Squires was taken to a triage and treatment area but was pronounced dead at 4:04 p.m., while Lopez was placed in restricted housing at the prison pending investigation by CDCR officials and the Marin County District Attorney’s Office.

At left, Mark Squires, 70, was found dead in his cell Tuesday at San Quentin Rehabilitation Center in what officials are calling a homicide. Gustavo Lopez, 36, (at right) Squires’ cellmate, has been placed in restricted housing pending an investigation. (CDCR via Bay City News)

Squires came to San Quentin in 2000 on a life sentence from Riverside County for lewd or lascivious acts with a child under 14 with force or violence. Gustavo also came from Riverside County, arriving in 2020 on a sentence of more than 13 years in prison for kidnapping, false imprisonment and other charges, according to the CDCR.

Dan McMenamin is the managing editor at Bay City News, directing daily news coverage of the 12-county greater Bay Area. He has worked for BCN since 2008 and has been managing editor since 2014 after previously serving as BCN’s San Francisco bureau reporter. A UC Davis graduate, he came to BCN after working for a newspaper and nonprofit in the Davis area. He handles staffing, including coaching of our interns, day-to-day coverage decisions and management of the newswire.