In front of a courtroom overflowing with spectators, a jury heard closing arguments yesterday in the trial of a man accused of killing a Metro Transit Police officer.

The audience included many transit officers, some of whom knew the victim, Marlon F. Morales, who was shot in the face in June 2001 at the U Street/Cardozo Metro station in Northwest Washington. They watched intently as prosecutors and defense attorneys debated whether Walter O. Johnson is the killer.

The jury is expected to begin deliberations today after D.C. Superior Court Judge Ann O'Regan Keary delivers final legal instructions.

Morales, 32, a native of Guatemala, was a rookie officer from Virginia who was married and had three children. He was the second transit officer killed in the line of duty, shot after coming to the aid of a Metro employee who was having trouble with a fare evader.

Over more than six weeks of testimony, the jurors heard from 54 witnesses and saw dozens of pieces of evidence. If convicted of murder of a law enforcement officer and other charges, Johnson, 36, could face a life prison term.

Assistant U.S. Attorney June M. Jeffries began her closing arguments by challenging the defense's contention that Johnson, of Philadelphia, was not even in the Washington area the night that Morales was shot. The officer died three days after the June 10, 2001, attack.

Cell phone records of women who allowed Johnson to use their phones during a bus ride from Philadelphia to Washington the day of the shooting, along with the testimony of one of those women, show that Johnson was in fact in the area, Jeffries said.

Ballistics evidence shows that the same gun that killed Morales was used in the shooting of a man a few days earlier in Philadelphia -- a crime that authorities have linked to Johnson, who had recently been paroled after serving time for armed robbery.

That earlier shooting -- and the fact that Johnson had that gun when Morales stopped him for fare beating -- gave the defendant a motive to kill Morales, Jeffries said. If Johnson were arrested, he would be searched and Morales would find the gun. It wouldn't take long for the gun to be linked to the shooting in Philadelphia, she said.

"Walter Johnson had to get out of that situation the way he knew best, and the way he knew best was to use that gun," Jeffries said.

When he was arrested a few days later in Philadelphia during a traffic stop, Johnson had Morales's gun, which had been stolen during the shooting in the Metro station -- another link, prosecutors said, to the killing.

But the gun, like much of the critical evidence introduced by Jeffries and her co-counsel, David Gorman, does not directly tie Johnson to the crime. Johnson's attorneys, Renee P. Raymond and Yvonne Williams, of the D.C. Public Defender Service, have said the prosecution's case has significant gaps.

In her closing argument, Raymond highlighted the varying descriptions of the defendant offered even by people who claim to have had encounters with him in the hours before and after Morales was shot. She said identification is the key to the case.

No one saw the shooting, which took place about 9 p.m.

The first prosecution witness in the trial, however, challenged the defense argument. Fernando Ferguson, the station manager that night, testified that he confronted Johnson over trying to beat the fare. Ferguson testified that he spoke with Johnson for a short time before handing the situation off to Morales.

Sensing that Morales was well on his way to resolving the matter, Ferguson left to use the restroom, he testified. Then he heard a shot.

Raymond has pointed out that Ferguson's original description of the fare beater was of a man 5 feet 7 inches or 5 feet 8, and Johnson is 6 feet 1. She said he is mistaken and is tailoring his memory to suit the prosecution.