A parole hearing is scheduled Wednesday for "Onion Field" killer Gregory Powell, who was convicted of murdering a Los Angeles police officer south of Bakersfield in 1963 after abducting him and his partner.

The Los Angeles Police Protective League, which represents the LAPD's rank and file, is opposing parole for Powell.

"This vicious murderer has not yet paid his debt to society and should be forced to serve the maximum term of his sentence," Paul M. Weber, the LAPPL's president, wrote in a letter last week to the California Board of Prison Terms.

"We must never show any tolerance for the killing of police officers. Please send the clear message that the murder of police officers is unacceptable and all those who are guilty of it must expect the harshest possible punishment available under the law," Weber added in his letter.

Powell, now 76, was convicted of murdering 31-year-old LAPD Officer Ian Campbell in a crime detailed in former LAPD Officer Joseph Wambaugh's best- selling book, "The Onion Field," which was later made into a movie.

Campbell and partner Karl Hettinger were abducted at gunpoint by Powell and Jimmy Lee Smith on March 9, 1963, while the two officers were patrolling in Hollywood. The officers were driven to an isolated onion field, where Powell shot Campbell to death.

Hettinger was shot at as he fled the onion field, but managed to escape in the darkness. But Weber wrote that "the incident haunted him (Hettinger) the rest of his life."

The original convictions of Powell and Smith were reversed on appeal and they were retried and convicted again. Powell was initially sentenced to death, but that sentence was reduced to life in prison after the U.S. Supreme Court ruled that California's death penalty law at the time was unconstitutional.

Powell has repeatedly been denied parole.

Smith, released from prison in 1982, was in and out of custody several times before dying at a Los Angeles County jail facility in April 2007.