The board considered Powell's case of Powell during a Tuesday meeting.
Powell is serving a life sentence for kidnapping officers Ian Campbell and Karl Hettinger in 1963. He fatally shot Campbell in an onion field near Bakersfield, while Hettinger escaped. The incident was chronicled in the novel "The Onion Field."
The 77-year-old has terminal prostate cancer. Under state law, inmates are eligible for compassionate release if they have less than six months to live.
The union representing L.A. police officers urged the board to deny Powell's release.