A 22-year-old Fairfax County motorist was killed late Wednesday on Rte. 1 a few blocks north of Fort Belvoir when an occupant of a car he had just passed fired a rifle into his pickup truck, wounding him in the head.

Kevin McGann, a graduate of Mount Vernon High School who recently had returned from California, was driving his 1979 blue GMC pickup home from a friend's house in the Lorton area when the shooting occurred at about 11:30 p.m., according to police.

Witnesses told investigators McGann had passed a car also traveling toward Alexandria when one of the car's two occupants fired the rifle. Police said the blast hit McGann in the back of the head, causing him to lose control of the pickup. It veered off the highway and came to rest near a ravine about 175 feet from the road.

McGann was pronounced dead at the scene, not far from Woodlawn Plantation. His parents, who lived with him in a two-story town house about two miles from the scene of the incident, said their son had returned home last summer from a three-year stay in California and was working toward a Virginia electrician's license.

"I can't believe it, I just can't believe it," McGann's mother, Anna-Mae, said yesterday. A bookkeeper for a nearby lumberyard, which also employed her son as a clerk, McGann said she could think of no one who bore a grudge against him. "They must have been sideswiping him. My son was a safe driver. In fact, he drove slower than I did. He was so proud of his truck."

Fairfax police said they have few clues as to who did the shooting or why. According to his family, McGann returned home after work yesterday evening, ate supper with his family, took a shower and went out to visit a high school friend. He was returning home when he was shot.

Police are looking for the assailant's car, which witnesses described as an early 1970s, two-door reddish Dodge or Plymouth, possibly a Dodge or Duster, with a raised rear end.

McGann's death shocked his coworkers, who yesterday described him as a tall, quiet young man with curly brown hair and blue eyes, a worker more interested in electronics than the lumberyard work he had done since leaving his father's groundskeeping company at the end of the summer.

"He'd been away in California for the last three years, learning to be an electrician," said Joan Gilson, a neighbor. "He came home this summer; I guess he was homesick. He worked for his father's maintenance company and then at the lumber yard and was studying to get his Virginia electrican's license. He'd just come home from a hunting trip a week ago."

His supervisor at the lumberyard, Patricia Smith, called the entire Woodlawn stretch of Rte. 1, including the spot where McGann was shot, a dangerous area and said shopkeepers have been plagued by armed robberies and shootings there in recent years. "There's an awful lot of crime on this road. Every day Kevin's mother would bring a newspaper back from lunch and nearly every day there'd be a police report of another robbery or shooting or something along here."