SAN FRANCISCO (CBS SF) — A man who was sentenced to death for killing an off-duty police captain during an attempted robbery in Southern California was found dead in his cell at San Quentin State Prison, officials said.

Miguel Angel Magallon, 35, was found unresponsive in his cell Saturday morning, according to the California Department of Corrections and Rehabilitation.

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He was pronounced dead a half-hour later following after emergency life-saving procedures failed to revive him.

Magallon was a 20-year-old gang member in 2004 when he killed 53-year-old Capt. Michael Sparkes of the Los Angeles County Office of Public Safety who was on a bike ride when Magallon confronted and then shot him with an AK-47.

Sparkes was able to call 911 on his cellphone and managed to shoot Magallon with his handgun after Magallon and his accomplice drove back to the scene of the shooting to fire on Sparkes again.

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The 30-year veteran of the department left behind a wife, adult stepdaughter and a 7-year-old son.

Magallon had been on California’s Death Row since Oct. 26, 2009.

Last month, Gov. Gavin Newsom signed a moratorium on executions in the state, citing racial disparities in the 736 inmates on Death Row, the largest in the nation.

California hasn’t executed anyone since 2006 when Arnold Schwarzenegger was governor because of legal challenges to its lethal injection method.

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Since capital punishment was reinstated in California in 1978, 13 inmates were executed, 79 condemned inmates have died of natural causes, while 26 committed suicide. 11 other death row inmates have died from other causes, and five – including Magallon – are pending a cause of death.