Fairfax County police said a 47-year-old man was shot while he was sitting in a car in the Mount Vernon area Wednesday night, but they have little evidence that points to a suspect or motive in the killing.

Police said officers found the man, whose name is being withheld until his relatives are notified, in the driver’s seat of a car parked near San Leandro and Mendocina places about 11:30 p.m. Thursday.

Police responded to the scene, on a residential street in a garden-style apartment development, after a neighbor complained about the car’s revving engine, police said.

The man, who had been shot in the upper torso, was taken to a hospital and pronounced dead, police said.

“This is a real mystery,” said Mary Ann Jennings, a police spokeswoman. “We are following up on a few leads, but we have little solid information.”

Jennings said several witnesses were interviewed but they provided few clues about what might have happened. She said detectives were seeking the public’s help in solving the case.

Bud Walker, another police spokesman, said it did not appear that anything had been stolen from the man’s car, and the vehicle had not been involved in a crash before the shooting.

Jennings said the man who died was not a Fairfax resident.

Some residents in the area said they heard the car’s engine revving but saw nothing unusual. The slaying is the county’s third this year.