On the 40th day after the murder of Ali Akbar Tabatabai, the exiled Iranian diplomat shot dead on the doorstep of his Bethesda home in July, solemn relatives and friends gathered at his Virginia grave yesterday to pray for his soul.

The 40th day holds special significance in the Islamic religion, as the day when the deceased's family and friends eat together and pray for his spirit. About 15 persons assembled yesterday afternoon at the tree-lined Islamic Garden in the National Memorial Park in Fairfax County.

The mourners, dressed in black with the women wearing veils, were an insecure cluster as media cameras clicked and whirred whenever they recited prayers from the Koran -- the Islamic holy book -- over the gray metal marker of Tabatabai's grave.

Beyond the garden, in a stark reminder of the threat of further reprisals against critics of the Ayatollah Ruhollah Khomeini, Fairfax County policemen monitored the crowd, using binoculars to scan the area. There were no incidents.

The slain man's twin brother, Mohammad Reza Tabatabai, walked among the assemblage shaking hands, accepting the more public role that his brother's death thrust upon him. But he remains troubled.

"My life will never be the same," Tabatabai said. "We are not going to some places. I am worried for the physical security of my family."

"I am learning," Tabatabai said, referring to the fact that his main concern now is to continue his brother's work. "I want to make sure that the freedom foundation [Iran Freedom Foundation] that my brother founded continues its work. I am trying to do all that I can."

While praising local police for their assistance, Tabatabai said that "we have never had police protection." Of federal authorities, he declared: "They have done nothing. They wait until a shooting and then they come out and investigate."

He said the fact that federal officials believe the man suspected of being the gunman -- Daoud Salahuddin -- has returned to Iran adds credence to the notion that Iran "supported [the killing] as an act of terrorism."

"The U.S. is the world's leading free nation. It is a disgrace to jeopardize this," Tabatabai said, "I ask, how free are you . . . I am not satisfied. It is an election year. They want to cool it down. The death of my brother was an act of terrorism."

On Friday, the Justice Department announced that it was dropping federal civil rights charges against Horace Anthony Butler, who is believed to have participated in the Tabatabai murder by helping obtain a postal jeep on the afternoon of the slaying. Butler still faces a Maryland state charge as an accessory to murder before the fact and was transported from Baltimore to the Montgomery County Detention Center Friday. A bond hearing will be conducted Tuesday morning, said Montgomery county police spokesmen.

Federal sources indicated yesterday that the U.S. Attorney's case against Butler was dropped because of a federal legal loophole that prevents prosecution of suspects if the victim is not a U.S. citizen.

FBI sources said they felt that the decision to drop charges might hamper their case. But FBI spokesman Paul Cundiff reaffirmed that Tabatabai's case "will remain a top priority case of the FBI and we will follow up any and all leads."

A third suspect, Tyrone Anthony Frazier, charged under Maryland state law as an accessory to murder both before and after the fact, was indicted last week by a Montgomery County Grand Jury.

Yesterday, after Tabatabai and his wife knelt before the grave that was decorated with red roses, carnations and chrysanthemums, Tabatabai said that the IFF headquarters will be moved from his brother's home to a more secure place.

Although he now acts as spokesman for the IFF, he said that the eventual leadership of the organization was not in his hands.

"The IFF is incorporated and has its own board of directors," Tabatabai said. "It is for them to decide.