LOS ANGELES (Reuters) - A California inmate on death row convicted of shooting to death a high school vice principal at his home during a robbery died of unknown causes on Thursday, prison officials said.

Gilbert Rubio, 55, was found unresponsive during a morning security check of his single-inmate cell at San Quentin State Prison north of San Francisco, the California Department of Corrections and Rehabilitation said in a statement.

The Marin County Coroner will determine the cause of death.

The state has 747 convicts on death row but has not put an inmate to death since 2006 due to court challenges. Since 1978, when it reinstated the death penalty, California has executed 13 inmates while 70 condemned prisoners have died of natural causes.

In 2000, Rubio was convicted of the 1998 robbery and first-degree murder of George Blackwell, prison officials said.

(Reporting by Alex Dobuzinskis)