A D.C. Superior Court grand jury has concluded that U.S. Park Police were justified in the fatal shooting last month of an antinuclear activist who threatened to blow up the Washington Monument, U.S. Attorney Stanley S. Harris said yesterday.

Harris said the six-week investigation into the shooting of 66-year-old Norman Mayer, killed in a barrage of bullets as he drove away from the monument toward the White House, produced "no evidence of wrongdoing" by the officers and Park Police officials involved in the shooting.

Harris said "no basis exists for the filing of any criminal charge" and that "the matter is now closed."

Mayer, who claimed he had 1,000 pounds of explosives in a van he drove to the foot of the monument, kept nine persons trapped there for five hours before releasing them. He kept police at bay for another five hours, saying he had a remote control device that could ignite the explosives. No explosives were found in the panel truck.

In a telephone interview yesterday from his home in California, Mayer's brother, Aubrey, said that he could "certainly see the grand jury's point of view" in reaching its conclusion. But Mayer said he still believed there were "other ways" the police might have stopped his brother short of killing him, such as shooting out the tires of the truck.

"I told them early on that he didn't have explosives . . . that it was a bluff . . . that he was not the type of person to have dynamite," Mayer said.

The police were under orders to shoot at the engine block to try to stop the van, according to a Park Police spokeswoman.

The investigation showed about a dozen bullets struck the engine, tires and and cab area of the truck.

Mayer was not hit directly by a bullet, D.C. Medical Examiner Dr. James Luke said yesterday, but rather by multiple fragments of bullets and parts of the van that ricocheted in the cab during the brief fusillade. Some fragments hit him in the arm, the face and in the head, Luke said. The fragments that hit him in the head caused the fatal injuries, Luke said.

U.S. Park Police Chief Lynn H. Herring, said in a statement yesterday that "The decisions that had to be made that day were extremely difficult and the grand jury findings did uphold those necessary actions."

Prosecutors said yesterday that no one had requested a grand jury investigation of the incident, but that the U.S. attorney's office, as a matter of longstanding policy, investigates all fatal shootings involving police.