The FBI has begun a preliminary investigation into last week's slaying of a member of the Guardian Angels by a Newark, N.J., policeman, the Justice Department said yesterday.

The FBI is reviewing official records in the case, said spokesman John Wilson. The next step would be for agents to conduct interviews, but Wilson declined to say whether the FBI had started interviews.

Official records in the case reportedly include two conflicting autopsy reports as to the location of the policeman, Milton Medina, when he shot and killed Frank Melvin, 26, a member of the Guardian Angels anti-crime volunteer organization.

The Guardian Angels have said the bullet that killed Melvin was fired from ground level without provocation. The Newark police said the bullet was fired by Medina from a rooftop because he thought Melvin was attacking his partner.

On Monday, New Jersey Attorney General James R. Zazzali decided against a separate state investigation of the slaying.

Wilson said the FBI began the investigation on its own initiative and "as far as I know we have not yet received any requests" from the Guardian Angels for an investigation. The group's founder, Curtis Sliwa, said earlier this week that the Angels would march on Washington to demand a Justice Department investigation.