A state parole board panel voted Friday to recommend that Jesus Cecena, convicted of the 1978 killing of San Diego police Officer Archie Buggs, be paroled.
The issue now goes to Gov. Jerry Brown, who has several months to make his decision.
Brown last year rejected a similar recommendation from a parole board panel.
"We'll fight this decision with everything we've got," said San Diego County Dist. Atty. Bonnie Dumanis. San Diego Police Chief Shelley Zimmerman urged the public "to join me and petition Gov. Brown to veto this terrible decision."
Buggs, 30, an Army veteran who served during the Vietnam War, had pulled over Cecena -- then 17, now 54 -- for a traffic infraction. Cecena, handed a gun by a passenger and fellow gang member, came out firing.
Buggs died at the scene. Cecena was arrested within hours.
After a similar hearing last year, one of the panel members said Cecena was eligible for parole because he "does not pose an unreasonable risk of danger to society or a threat to public safety."
Cecena's attorney, Tracy Lum, issued a statement noting that Friday's panel found her client "fully rehabilitated."
"It is my hope that when the governor reviews the board's decision," Lum said, "he will also judge Mr. Cecena on who he is now and not solely on the horrible crime he committed as a juvenile."
Dumanis attended Friday's hearing to argue that Cecena should remain in prison. The "cold-blooded murder" of Buggs devastated his family, the city and the Police Department, she said.
"Criminals who kill police officers are different," Dumanis wrote in a letter to the parole board. "Cecena has proven this by steadfastly refusing to acknowledge full responsibility for this most horrific act."
The two-person panel, meeting at Valley State Prison in Chowchilla, decided that Cecena meets the standards of a new law, SB 260, that says the parole board must give "great weight to the diminished culpability of juveniles" and also consider an inmate's "maturity and rehabilitation in prison."
Lum said that despite what prosecutors assert, her client has taken responsibility for killing Buggs and has "worked diligently to rehabilitate himself."
The law and the Legislature, she said, recognize "that incarcerating a person like Mr. Cecena, who committed his crime as a juvenile, is cruel and unusual, and such persons ... have a constitutional right to a meaningful opportunity to be released from prison."
A central question was whether, after Buggs was already gravely wounded and lying in a gutter, Cecena walked over to him and shot him again, in the head.
Prosecutors said the evidence in the case proves that Cecena fired a final, "execution-style" shot. Lum said the evidence does not prove that. Cecena has denied firing a point-blank shot.
Under SB 260, 216 inmates sentenced to long terms for crimes committed as juveniles have been judged eligible for parole, subject to the governor's approval. An additional 477 have been rejected.
Dumanis and Deputy Dist. Atty. Richard Sachs presented evidence during the two-hour hearing including pictures from the crime scene and autopsy, a statement from Buggs' mother and a video of then-Mayor Pete Wilson's eulogy at Buggs' funeral.
Zimmerman had submitted a letter opposing parole for Cecena and insisting that "justice demands that he be made to spend every day of his life in prison."
Lum presented testimony to show that Cecena has taken educational courses in prison and become a mentor to younger prisoners about avoiding gangs and substance abuse, and that he has job offers and a support system if he is released.
Cecena's co-defendant, Jose Arteaga, who was 20 when Buggs was killed, remains in prison but is due for a parole hearing within the next year.