A 17-year-old senior at T.C. Williams High School in Alexandria was shot and killed late Saturday night while driving on Route 1 in the Mount Vernon area during what may have been a dispute with some pedestrians, Fairfax County police reported.

Robin M. Anderson, of the 100 block of East Randolph Avenue, who would have been 18 on Wednesday, was shot about 10:45 p.m. near Ladson Lane, police said.

The circumstances of the shooting remained unclear last night.

Officers said the killing came after Anderson and others in his vehicle became involved in an argument with pedestrians. But one of Anderson's passengers said in an interview that he heard no words exchanged and that the shots came without warning.

"I didn't hear anyone say anything," said the youth, who asked not to be identified. He called his friend "a nice guy" who had a passion for electronics and "stayed out of trouble."

"He wasn't arguing," Anderson's grandmother said last night. "It was a senseless killing."

A neighbor said she had a high regard for Anderson, whom she termed polite and "real nice." She called his family "just good people."

The youth's friend said that he, Anderson and two others were out driving Saturday night, aware that Anderson had to return home in time for his family's midnight curfew, when they decided to make a stop at a Route 1 convenience store.

They were heading south on Route 1 toward the store when "some guys ran across the street," the friend said. "We didn't pay any attention." Then, he said, he heard shots, and Anderson "fell over in my lap."

He said he looked to where the shots seemed to come from and "saw those guys running." The friend, who stopped the car, said he did not recognize them.

Anderson's grandmother said she had imposed the curfew, which she said prevented him from going to see a movie Saturday night. "I had told him to come back," she said. "Maybe if he had stayed . . . "