A traffic dispute between two motorists on Rockville Pike turned deadly last night when one driver, an off-duty deputy U.S. marshal, fatally shot the other in the parking lot of a busy shopping center, police said.

The federal law enforcement officer, who was driving with his family in a sport-utility vehicle, exchanged words about 8:30 p.m. with a young man driving alone in a red Chevrolet Camaro with New Jersey tags, said Capt. John Fitzgerald, a Montgomery County police spokesman.

The two drivers pulled into the Mid-Pike Plaza, where they got into a fistfight, Fitzgerald said.

"Preliminary information is that the deputy marshal tried to defuse and disengage, to calm it down and settle it, and wanted to get the local police involved," Fitzgerald said.

He said the young man got back into his car and drove toward or near the off-duty officer, who then opened fire, shooting multiple times. Officers found nine shell casings at the scene. The young man was taken to Suburban Hospital, where he was pronounced dead a short time later.

Fitzgerald could not confirm reports that the dispute stemmed from a fender bender.

A witness at the scene offered a different account of the incident at the Rockville shopping center where some had gone shopping for Halloween.

Eugenia Hull of Silver Spring said she and her sons, ages 12 and 15, were walking through the parking lot of the center, in the 11800 block of Rockville Pike, when they came upon the men having a dispute.

She said that the off-duty deputy stood in front of the Camaro with his gun drawn and that he ordered the young man out of the car.

She said the deputy marshal shouted, "Get out of the car, or I'm going to shoot you again." He also said, "that's all the ID you're going to see," she added.

The deputy then threatened to shoot if the young man moved his car, Hull said.

"Then I watched the Camaro move around" the deputy, she said, and "I heard pop, pop, pop." She said the car jumped the sidewalk and crashed into the building.

"I feel it's so wrong for him to shoot this kid," she said. "I can't believe he shot him. I heard people say they had an altercation. How does that give him the right to shoot him?"

Fitzgerald said the incident is under investigation.

Attempts to get comment from the U.S. Marshals Service were unsuccessful. The deputy was treated at a hospital for minor injuries, officials said.

The names of the officer and the other driver were not immediately released.

Montgomery County investigators at the scene of the shooting on Rockville Pike. Police said the shooting followed a fistfight.