A Montgomery County grand jury has indicted Horace Butler, a 35-year-old carpenter, on charges of conspiracy and being an accessory in the assassination of Ali Akbar Tabatabai, a prominent Iranian exile who was shot to death in the doorway of his Bethesda home last July.

Butler, who has been held without bond at the county detention center since his arrest July 23, was indicted late Thursday on two counts of being an accessory before and after the fact of murder for his role in the fatal shooting of Tabatabai, a staunch leader of anti-Khomeini exiles in the United States.

Butler, who gave an address of 4921 LaSalle Road, Avondale, at the time of his arrest, was also charged with one count of conspiracy to commit murder with Daoud Salahuddin, the alleged trigger man FBI officials now believe to be in Iran.

According to the indictment, Butler aided and counseled Salahuddin in connection with Tabatabai's slaying and afterwards "did harbor, comfort and assist" the assassin.

At the time of his arrest, police reports said Butler was the driver of a pick-up truck that took Salahuddin on July 22 to a rendezvous with a mailman whose jeep-type mail truck the assassin had arranged to rent. For five hours on the afternoon Tabatabai was killed, police said Butler was driving the mailman, 31-year-old Tyrone Anthony Frazier, around Baltimore.

Frazier originally said he had been kidnaped on the day of the killing, but he changed his story after a night of questioning at Montgomery County police headquarters.

Frazier is currently in the Montgomery detention center awaiting trial, scheduled for Nov. 17, on two counts of accessory to murder and one count of making a false statement to a police officer.

No trial date has been set for Butler.