A man awaiting execution for the rape and murder of his 15-year-old stepdaughter has died of unknown causes.

The California Department of Corrections and Rehabilitation said Thursday that 54-year-old Leon Chauncey Cooper died Wednesday at a hospital near San Quentin State Prison, home to the state's death row.

The cause of death is awaiting an autopsy.

He was sentenced to death in 2001 by a Sacramento County jury for the 1998 rape and murder of LaRhonda Johnson, a high school sophomore.

Cooper previously had been convicted in 1996 of sexual battery on Johnson's 18-year-old sister.

Sixty-six condemned inmates have died from natural causes since California reinstated capital punishment in 1978, while 14 have been executed. Another 23 committed suicide.

Death row now holds 752 inmates.