A federal investigation into the fatal shooting of a robbery suspect by a Houston police officer early last year has raised new questions about the conduct of the city's controversial police department.

Police Chief Harry Caldwell has confirmed that an unloaded 22-cal. revolver the suspect allegedly pointed at an officer had been in the department's possession after it was recovered from the scene of a 1964 suicide.

Caldwell said police records show the gun was to have been destroyed in 1968, a normal police procedure. "It is pretty damn obvious to me now it was not destroyed," he said.

The case has aroused suspicion that the weapon was a "throw down gun" planted at the shooting scene by police to justify their use of deadly force.

Some local defense attorneys have charged the district attorney's office here was less than vigorous in ivnestigating the shooting, a problem they say is common in cases of alleged police misconduct.

"For years it was traditional that any police officer who shot someone would be no-billed" and no action was taken, said Dick DeGuerin, past president of the Harris County Criminal Lawyers Association. "Although that's changed somewhat it's not because of the grand jury but because of the public's outcry."

Coming on the heels of the federal court conviction last month of three former Houston police officers for conspiring ot violate the civil rights of a prisoner who drowned while in their custody, this case has prompted appeals from minority group leaders for creation of a civilian police review board.

Two of the three officers convicted in federal court had also been indicted on state murder charges, but had subsequently been convicted of criminally negligent homicide, a lesser charge. Each was sentenced to one year on probation.

The Mexican-American Bar Association here and Councilman Judson Robinson, Jr., the city's only black elected official, say present investigatory safeguards are inadequate.

Mayor Jim McConn, however, opposes review of police action by civilians not familiar with police work. "I think it gets into a very political thing otherwise," he said.

The present federal investigation results largely from the efforts of John Webster, a Shreveport, La., homebuilder and the father of the shooting victim, Randall Webster, 17.

Webster said he contacted the U.S. attorney's office in Houston last fall after a Harris County grand jury cleared the officers involved in his son's death.

"I knew that Randy wouldn't commit suicide, and that's what pointing an unloaded gun at someone amounts to," said Webster. He says police and state prosecutors made only perfunctory efforts to investigate the case.

Webster's belief that his son was unarmed is supported by Billy Junior Dolan, a cab driver who said he saw Randall Webster attempt to surrender to police after leading them on a high-speed chase in a stolen van.

Dolan said two officers pulled Webster from the van, one held him face-down to the pavement, and a few seconds later he heard a muffled gun shot, "like when you shoot a watermelon."

Dolan's account of the shooting is backed up by another witness. A third man supports the story of three officers who were at the scene, who say Officer D. H. Mays shot Webster when he emerged from the van with a pistol in his hand.

District Attorney Carol Vance said his staff relied on a report from the police homicide division in presenting the case to the state grand jury.

The report said the pistol found at the shooting scene could not be traced beyond its shipment to a local discount store in 1964.

"Of course, hindsight is 20/20 but in light of the witnesses we had, we didn't have reason to do an independent investigation of the weapon," Vance said.

In future such cases, his staff "may do a little more leg wokr" rather than simply rely on police reports, Vance said, and he may bring perjury charges against the officers who testified before the grand jury.

However, he will await the outcome of the federal investigation before reopening the case. "We would not duplicate any substantive charges that may come from that investigation," he said.