Montgomery County investigators are trying to determine whether an off-duty deputy U.S. marshal was performing in his capacity as a law enforcement officer when he shot and killed a motorist after a confrontation on Rockville Pike on Thursday night, authorities said.

As authorities weighed whether criminal charges should be brought against the deputy marshal, Arthur L. Lloyd, 53, new details of the altercation that led to the fatal shooting emerged from dozens of witnesses interviewed by police.

Law enforcement sources said yesterday that some witnesses said Lloyd argued and fought with Ryan T. Stowers for some time before Lloyd pulled out a badge and announced that he was a federal officer.

Those sources said Stowers was shot in the ankle during the altercation, which began shortly before 8:30 p.m. in the Mid-Pike Plaza parking lot near Rockville. The 20-year-old returned to his red Camaro, refused Lloyd's order to get out and was fatally shot in the back as he apparently tried to drive away, according to police and witnesses interviewed by The Washington Post.

If authorities conclude that Lloyd fired the shots while acting in a law enforcement capacity and in justifiable self-defense, they could decide that criminal charges are not warranted. Law officers have more legal latitude than do civilians to shoot if they believe their lives or the lives of others are in danger.

Lloyd, a 28-year veteran of the U.S. Marshals Service, was beaten "pretty badly," and his thumb was broken during the fight, one source said.

Sources said there is considerable disagreement among prosecutors, and between police and prosecutors, as to whether Lloyd should be arrested now or whether a Montgomery County grand jury should be asked to determine whether the shooting was justified.

The long-standing policy of Montgomery State's Attorney Douglas F. Gansler is to refer all cases involving the use of deadly force by police officers to a grand jury before filing any criminal charges. If the grand jury ruled that the shooting was unjustified, criminal charges could then be filed. If the grand jury ruled that the officer was justified, criminal charges generally would not be filed.

Some involved in the discussions are arguing that the evidence shows that Lloyd did not identify himself as a law enforcement officer until long after the altercation began. Therefore, their argument goes, Lloyd should be charged as any other citizen would.

Police spokesman John Fitzgerald said yesterday that Lloyd had not been arrested because authorities are still sifting through as many as 50 witness accounts of the shooting and want to make sure they have all the evidence.

"The police department will make sure justice is done," Fitzgerald said. "Nobody is above the law. We're not going to give somebody a break just because they carry a badge."

Lloyd, who has several children and has lived in the area for many years, is not considered a flight risk, Fitzgerald said.

"If we decide to arrest this person, we know exactly where to find him," he said.

The number of cases in which police face criminal prosecution for shootings is very small, said Bill Geller, a Chicago author of several major studies on the use of force by police.

Geller said most cases involving the use of deadly force turn on whether the officers were in "reasonable" fear of their lives. If the facts of the case show that Stowers intended to use his car as a deadly weapon and run Lloyd over, it is possible that could be viewed as reasonable fear, Geller said.

Witnesses who have spoken with The Post agreed that Stowers tried to drive away, but they did not agree on whether he moved in reverse or tried to swerve around Lloyd.

"If the facts turn out to be that the driver wasn't trying to run down the deputy but simply trying to get away, the question then becomes, what set of facts justify a shooting in order to prevent the escape?" Geller said. "In some situations, a police officer would be justified in using deadly force in preventing the escape of someone suspected of a serious felony."

Lloyd was placed on paid administrative leave yesterday pending the completion of Montgomery County's investigation, said David Sacks, an agency spokesman.

David Sanford, a lawyer representing Lloyd, declined to comment yesterday.

Staff writer Amit R. Paley contributed to this report.

Ryan Stowers was fatally shot on Rockville Pike.