John Levae Post, 42, was found unresponsive at 2:20 p.m. and was pronounced dead at 2:52 p.m., the Department of Corrections and Rehabilitation said. The death is under investigation.
Post was convicted of the first-degree murder in connection with the Sept. 22, 1996, drive-by shootings deaths of Aaron Johnson, 17, and 23-year-old Larry Logan.
He was sentenced Dec. 17, 2001, and was placed on San Quentin's Death Row on Dec. 26, 2001, prison officials said.
Since 1978, 52 condemned inmates have died from natural causes, 18 committed suicide and 13 were executed at San Quentin, according to the Department of Corrections and Rehabilitation. One inmate was executed in Missouri and five Death Row inmates died of other causes, prison officials said.