A federal jury convicted a member of a secretive militant Muslim group yesterday of charges carrying a maximum penalty of life plus 53 years for threatening officials investigating the 1980 assassination of Iranian dissident leader Ali Akbar Tabatabai.

The jurors deliberated for a total of four hours late Thursday and again yesterday morning in U.S. District Court in Alexandria before returning their guilty verdict on all nine counts against Robert Lee Johnson, also known as Khalid Abdul Latif.

Prosecutors alleged Johnson and a codefendant, Melvin Davis, were members of the Islamic Guerillas of America, a group that has been tied to the Tabatabai slaying.

Davis, also known as Muhammad Abdul Malik, had been on trial with Johnson. However, on Wednesday he pleaded guilty to six charges in the nine-count indictment, and appeared later as a key defense witness.

The charges against Johnson, 34, of Hyattsville, included conspiracy to murder federal officials, assault and using the mails to send written threats to two assistant U.S. attorneys, a federal judge and Sen. Strom Thurmond (R-S.C.).

The government also accused both men of participating in two incidents in August 1980 in which shots were fired into the Fairfax County homes of Norman Carlson, director of the U.S. Bureau of Prisons, and an aide, James Meko.

Judge Richard L. Williams set sentencing for Johnson and Davis for Jan. 15.

Defense testimony in the three-day trial began Thursday morning with Davis retracting earlier statements he had made to the FBI that implicated Johnson in the Carlson and Meko shootings and other threats; instead, Davis took the blame for the conspiracy himself.

Davis, a convicted felon who said he met Johnson in prison in Leavenworth, Kan., in the mid-1970s, testified the two men were on opposing sides early last year in a bitter struggle for political control of the Islamic Center at 2551 Massachusetts Ave. NW.

Davis -- claiming he succeeded the accused slayer of Tabatabai, Daoud Salahuddin, as chief operations officer of a militant group trying to seize control of the center -- said he was ordered in mid-1980 to assassinate Johnson because "he was too much of an opposing force."

Davis said he was unable to carry out the execution because of his friendship with Johnson and Johnson's family. Rather, he said, he decided to try to win Johnson over by falsely implicating him in the shootings and threatening letters.

"I felt if enough pressure was put on him by federal authorities, he'd get angry and come to my side," Davis said.

Prosecutors accused Davis of changing his story because he feared retribution in prison. With Davis on the witness stand, prosecutor Theodore Greenberg produced a sworn statement Davis signed in May as part of a plea bargain in which he admitted guilt in the Carlson and Meko shootings and named Johnson as an accomplice.

Greenberg and Davis stood face-to-face in the courtroom shouting at each other at one point until Williams ordered them to separate.

Hassan Jeru-Ahmed, assistant to the director of the Islamic Center, said in defense testimony that Johnson had been involved in blocking attempts at a takeover by Muslim militants. He acknowledged under cross-examination, however, that he had seen Davis at the center only once.

In his closing statement, defense lawyer R. Kenneth Mundy asked for an acquittal for Johnson, saying Davis "lied whenever it was convenient. . . . This was a man who had a lie for whatever occasion was present."