Five, not three, men were involved in last spring's daring, drug-related murder plot at the D.C. prison facility in rural Lorton, and two of them still are at large, a star prosecution witness testified yesterday in court.

Federal and local authorities previously had said only three intruders were involved in the plot that left an inmate shot to death, and that all three were arrested hours after the shooting.Prosecutors said yesterday they now believe there were five.

The witness, John Elbert Landon, 30, testified in Alexandria federal court that he, two suspects who went on trial yesterday and two other men -- one known to him by a nickname and the other only by a "code name" -- crept through the muddy countryside in the predawn hours last May 23 as the plot unfolded at the city's lightly guarded minimum security facility in Fairfax County.

Two men known as "Scoobie" and "R-3" participated in the break-in, Landon testified, along with himself, Edward Ford (who formerly called himself Edward Ford Sharrieff) and Germain P. Stoddard.

Ford and Stoddard are on trial on a charge of first-degree murder in the shooting death of Lorton inmate Douglas M. Boney. Landon, who pleaded guilty to assault with intent to murder in the case, is a chief witness against them.

Landon testified that Ford stood over the already wounded Boney and said, "You're a ganster, now die like a gangster."

Then, he said, Ford shot Boney twice in the chest with a .38-caliber pistol.

"Boney sounded like a chicken dying," said Landon, making a coughing sound and holding his right hand out to portray what he said was Boney's "last breath."

Landon, who wore a three-piece suit, testified during his two hours on the witness stand that he and the other defendants had agreed after their arrest to lie about their numbers and activities. "I lied to my wife," he added.

Assistant U.S. attorney Joseph Aronica said outside court yesterday that law enforcement officials are baffled as to the identities or whereabouts of "Scoobie," believed to be a relative of Ford, and "R-3."

Ford, 32, of 16 S. Van Buren St., Alexandria, and Stoddard, 24, 409 Upshur St. NW, have pleaded innocent.

The shooting was related to a drug war that had begun between Ford and Boney after their drug partnership broke up earlier this year, Landon told the jury in Judge Robert R. Merbige Jr.'s court.

Federal officials said at the time of the shooting that the drug operation included extensive sales of narcotics within the prison itself.

During opening statements yesterday, Assistant U.S. Attorney Joseph Aronica told the jury that another figure in the case was D.C. Corrections Department counselor Robert (Pappy) Gault. Aronica did not expand on his statement, and Gault has been accused of no wrongdoing.

FBI officials have said that the 1980 Cadillac Seville allegedly used by Ford in the incident was registered to Gault's home address in Lorton, though not to Gault himself.

Gault, who earns $20,000 annually in his city job, has said that the car was registered to his address without his knowledge and that Boney was a friend.

Security measures at the U.S. courthouse on Washington Street were unusually tight yesterday. The trial is being held in a windowless third-floor courtroom, with as many as eight deputy U.S. marshals stationed inside. Spectators and witnesses must pass through a metal detector before entering.

Landon told Assistant U.S. Attorney Karen P. Tandy yesterday that he agreed to help Ford and the others because he was afraid of Ford, to whom he owed $1,000. "He said he'd kill me," Landon testified, "and then he'd kill my wife."

Landon claimed he did not carry a gun on the night of the shooting, and did not go inside Lorton's Dormitory 16, where Boney was staying, when the other four men ran inside. Instead, he watched the events through a window, and then ran to his car parked nearby, he testified.

Shortly before approaching the dormitory, Landon said, the five men "squatted down in a shack" nearby and were assigned "code numbers" by Ford, who was afraid his name would be recognized by some of the inmates or guards in the facility. Ford called himself R-1, "Scoobie" became R-2, the other man unknown to Landon became R-3, Landon was R-4, and Stoddard R-5, Landon testified.

Then the other four men, armed with four pistols and a pump shotgun, ran into the building, and one yelled, "We were sent by the Islamic Center to kill Boney and John D.", according to Landon. The reference to the Islamic Center was not explained.

Officials have identified "John D." as John D. Irby, a convicted murderer serving time at Lorton. He was unharmed.

Landon testified that one of the four men, possibly "Scoobie," shot Boney once before Ford followed the wounded man outside and fired twice. According to medical testimony, two bullets, fired from no farther than a foot away, caused Boney's death.

Ford also pistol-whipped the dying Boney more than a dozen times, Landon testified.

Lorton guard Henry L. Brown testified that a hooded man he identified as Ford held a shotgun to his head before leaving to chase Boney.

Defense attorneys attacked Landon's credibility in opening arguments and cross-examination. Ford's attorney, P. David Gavin, said prosecutors and Landon had written "a play . . . a script to please you."

Landon ended his testimony by stating that when he and Stoddard were trying to escape, they had to stop for gas at a Lorton Amoco station. When they got back on the road, with Stoddard at the wheel, they were headed in the wrong direction. Ford, meanwhile, had lost his car keys, and had to use a rock to break open his car window so he could use his car telephone to call a girl friend for a ride, Landon testified.

Ford was arrested as he walked near a road, and Landon and Stoddard were arrested as they drove into Washington.