Powell then fatally shot Campbell, but Hettinger managed to escape.
In a letter to the State Parole Board, the police union, the Los Angeles Police Protective League, urged officials not to grant Powell parole.
"This vicious murderer has not yet paid his debt to society and should be forced to serve the maximum term of his sentence," said Paul Weber, president of the Los Angeles Police Protective League, in the letter.
"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.
The crime was the subject of a best-selling book and later a film.
Powell is now 76 years old.
CNS contributed to this report.