An accomplice of a man convicted decades ago of killing 14 young men and boys then dumping their naked bodies along Southern California freeways has been killed in prison, officials said Thursday.

Gregory Miley, 54, died Wednesday after being attacked two days earlier by another inmate in a Mule Creek State Prison exercise yard, state corrections department spokesman Joe Orlando said.

He was sent back to his cell after an examination at the prison’s medical facility, became unconscious about 90 minutes later and died at an outside hospital after he was taken off life support.