(CNN) -- A 68-year-old California inmate on death row in San Quentin State Prison was found dead hanging in his cell, authorities said Tuesday.

Authorities are investigating the suicide of condemned inmate James Lee Crummel and declined to provide the circumstances of the hanging, which happened Sunday afternoon in the rear of his single-occupant cell, said Lt. Sam Robinson of the California Department of Corrections and Rehabilitation.

A corrections officer had last observed Crummel 15 minutes before he was found dead, Robinson said. The officer was doing an inmate count before dinner when he found Crummel's body, Robinson said.

Crummel had not been under suicide watch, Robinson said.

An autopsy will be conducted, he said.

Crummel went to death row shortly after being sentenced to death on July 9, 2004, by a Riverside County jury for the April 13, 1979, kidnapping, sexual abuse and murder of 13-year-old James Wilfred Trotter, who disappeared on his way to school, authorities said.

The boy's remains were found in 1990, and the identity of the body was not confirmed until 1996, authorities said.

California reinstated capital punishment in 1978, and since then, 57 condemned inmates have died from natural causes, 20 have committed suicide, 13 have been executed in California, one was executed in Missouri and six died from other causes, authorities said. California has 723 convicts on California's death row.