Los Angeles (CNN) -- The California Board of Parole Hearings declined Tuesday to grant a compassionate prison release to Gregory Powell, the infamous "Onion Field" cop killer whose 1963 crime was chronicled in Joseph Wambaugh's best-selling book.
The ruling means that Powell, 78, who has terminal cancer with less than six months to live, will die behind bars, according to officials and a prison physician's prognosis.
"The conditions under which the prisoner would be released or receive treatment pose a threat to public safety," the board said in its ruling.
"Powell's release would pose a public safety risk due to his history on noncompliance and lack of cooperation with prison rules, his failure to follow recommendations made by the board to render suitable for parole, his current physical abilities and the fact that he expressly does not wish to be considered (for compassionate release), and therefore, will likely be noncompliant upon release and would cause harm to be returned to prison, where he wishes to remain," the board said.
Powell is serving a life sentence with the possibility of parole for first-degree murder at the California Medical Facility in Vacaville, said Terry Thornton, a spokeswoman for the California Department of Corrections and Rehabilitation.
Prison officials have not disclosed Powell's illness because of state and federal privacy laws, Thornton said.
But the Los Angeles Police Protective League said Powell has cancer, and the police officers' union opposed a compassionate release, saying he should die in prison. Also opposing his release were Los Angeles County District Attorney Steve Cooley and relatives of slain Los Angeles Police Officer Ian Campbell, who Powell killed.
"To have released the man who kidnapped and callously executed Officer Ian Campbell would have been a travesty of justice," Cooley said in a statement.
Even though Powell has said that he didn't want to be compassionately released, he met criteria under a state law requiring that he be considered for such a recall of his sentence, Thornton said. A Corrections Department secretary designee in September recommended that Powell's sentence not be recalled, Thornton said.
"If an inmate is not on death row and is not serving life without parole and he's terminally ill and basically has six months or less to live as determined by a physician, then he meets the criteria under the law to be considered for compassionate release," Thornton said.
The state's 12 commissioners on the Board of Parole Hearings made their ruling on Powell's case during the board's monthly executive meeting in Sacramento.
Powell is among the longest-serving inmates in California's prison system, Thornton said.
More than 48 years ago, Powell and accomplice Jimmy Lee Smith kidnapped and murdered officer Campbell, 31.
On the night of March 9, 1963, Powell and Smith were driving around Los Angeles, looking for a liquor store to rob.
Campbell and his partner, Officer Karl Hettinger, pulled the two over in a routine stop. Powell, who was ordered out of the car, pointed a gun at Campbell's head. He and Smith disarmed both officers, took them hostage and drove to a remote onion field in Bakersfield, a town about 110 miles of north of downtown Los Angeles.
The officers were forced out of the car and ordered to stand with their hands above their heads. Powell said to them, "We told you we were going to let you guys go, but have you ever heard of the Little Lindbergh Law?"
"Yes," Campbell replied. Powell then shot him to death. Hettinger escaped, but the murder of his partner haunted him for the rest of his life.
Powell and Smith were sentenced to death in November 1963. Their sentences were commuted to life in prison with the possibility of parole in the early 1970s when the death penalty was declared unconstitutional.
Smith died at a California detention center in 2007.
The Little Lindbergh Law makes a kidnapping within the state a capital offense even if the victim is unharmed. It followed a federal law, nicknamed the Lindbergh Law, that made taking a kidnapped person across state lines a federal crime. That law was passed after the kidnapping and murder of the young son of aviator Charles Lindbergh in 1932.