SAN QUENTIN — A condemned murderer who was one of just 16 inmates on California’s Death Row to have exhausted his appeals died of unknown causes, prison officials have announced .

San Quentin officials are investigating the death of Fernando Belmontes, 56, but say there was no obvious cause. More details about his death earlier this month have not been released.

Belmontes was one of 16 condemned inmates — out of California’s nearly 750 — who had exhausted his appeals. As such, he was considered a top priority for execution.

Belmontes was sentenced to die at 20 years old, a year after he murdered 19-year-old Steacy McConnell during a 1981 burglary. It started when he and two others broke into McConnell’s home in San Joaquin County, just east of Lodi.

Belmontes, who was living in a halfway house at the time, bludgeoned McConnel 15-20 times with an iron dumbell, crushing her skull. In 1979, he had been convicted of being an accessory in a voluntary manslaughter, and he attacked his pregnant girlfriend months before the murder.

California has executed only 13 death row inmates since 1978, including the controversial 2005 execution of Stanley “Tookie” Williams, a Crips gang dropout convicted of a double-murder who’d written books to steer youth away from gang life. The most recent execution was in 2006, when Clarence Ray Allen was executed for organizing three murders while serving a life sentence for another murder conviction. Allen Spent 23 years on Death Row.

By contrast, 71 condemned inmates have died from natural causes, and 25 have committed suicide since 1978. In November, voters rejected a measure to overturn the death penalty, and passed a measure designed to streamline the execution process.

Belmontes’ death sentence was overturned in 2003, then reinstated in 2006. Belmontes lost his final attempt at a commuted sentence in 2010.

Prison officials say the coroner is conducting an autopsy to determine how Belmontes died.