One man was killed and six persons were injured in a multiple collision involving three trucks and four passenger vehicles that backed up traffic on both loops of the Capitol Beltway for almost five hours yesterday afternoon, Maryland state police reported.

The collision -- a chain reaction over six lanes of traffic -- started at 12:15 p.m. with a tractor-trailer driven by Laurence Contee, 35, of 5290 Marlboro Place, Capitol Heights. Contee was driving in the right lane of the Beltway's eastbound inner loop, near the Connecticut Avenue exit, when his truck suddenly veered from the lane for reasons police said are "inexplicable."

His truck swerved across the middle and left lanes, carrying with him two passenger cars driven by Paula S. Roberts, 34, of 1726 Pine Valley Drive, Vienna, and Linda Anne Benitez, 32, of 9632 Horizon Run Ride, Gaithersburg.

Contee's tractor-trailer then "vaulted" across the median, according to police, landing on its side in the left lane of the outer loop.

Barbara Thorne, 35, of 406 Lincoln Ave., Takoma Park, was driving in that lane, and hit her brakes as she saw the huge truck flying toward her.

Police said her car was hit from behind by a tractor-trailer driven by Raymond H. Harris, 52, of 9410 Gumtree Park, Capitol Heights. Contee's tractor-trailer crashed on its side, dumping an estimated 20 tons of gravel across the highway. As it skidded into the middle lane, it was struck by a tractor-trailer driven by Melvin Ray Plasters, 47, of Rte. 1, Ridgeway, Va. Contee's truck skidded further into the right lane and collided with a car driven by George Worthington Fredenburg, 44, of 8915 Connecticut Ave., Chevy Chase. Fredenburg was killed instantly, police said, and his car was demolished.

Contee, Thorne and Plasters were taken to Holy Cross Hospital where they are in stable condition after treatment for injuries. State police described Contee's injuries as "slight." Roberts and Benitez were taken to Suburban Hospital and later released.

By 12:28 p.m., when state police arrived, traffic on both sides of the Beltway was completely stopped. The Beltway's outer loop remained closed from Connecticut Avenue to Rte. 95 until 5 p.m.; some traffic resumed on the inner loop shortly after the accident.