A passenger in the truck that burst into flames following an accident on the Cabin John Bridge Wednesday died early yesterday of extensive burns.

The driver--the passenger's father--who apparently fell or jumped from the truck's cab to the Potomac River in the moments following the crash, was still missing and presumed dead.

As repair crews worked to replace torn guardrail and fix fire-scarred pavement, the bridge, which had been blocked for hours starting early in the Wednesday afternoon rush hour, slowly returned to normal, although miles-long backups plagued yesterday's morning rush and there was still a backup when all lanes were reopened at 3 p.m..

Maryland state police said yesterday the accident apparently occurred when the truck driver swerved to avoid an auto that made an improper lane change in the Virginia-bound lanes. The truck then hit another car, plowed through the bridge's guardrail and caught fire, according to police. The car that police say started the chain of events apparently left the scene without damage.

Peter Jay O'Donnell, 30, of Mason City, Iowa, who was a passenger in the 18-wheel tractor-trailer rig, suffered second- and third-degree burns over 95 percent of his body and died in the Medstar unit of Washington Hospital Center, where he was flown by helicopter following the 3:30 p.m. accident.

Police said originally that Peter O'Donnell was driving the truck, but yesterday they said the driver was his father, John Patrick O'Donnell, 57, also of Mason City, who is believed to have jumped or fallen almost 100 feet to the river.

The search for John O'Donnell included forays into the river by police divers Wednesday afternoon. The search was called off yesterday after sweeps of the area downstream by a police helicopter.

The driver of the car the truck hit, Robert A. Rich, 24, of Gaithersburg, remained in the Hospital Center, where he was reported in fair condition suffering from second- and third-degree burns over 10 percent of his body.

Maryland state police said they want to question the driver of a car described as a 1977 or 1978 silver-colored Ford LTD that may have prompted the truck driver to swerve when it changed lanes. The truck hit Rich's 1979 Toyota in the bridge's left lane and both then hit the guardrail, police said. About 100 feet of the railing was knocked out and the trailer's cab came to rest overhanging the river.

The fire that broke out was fed by fuel from one of the truck's tanks that broke open in the crash, according to Maryland state trooper Charles Ruby. The blaze, which spread to the truck's cargo of plastic covers for fluorescent light fixtures, proved persistent and resisted firefighters' efforts to extinguish it for several hours.

One of the Virginia-bound lanes on the bridge--formally known as the American Legion Memorial Bridge--was still closed during yesterday's morning commuter rush, which lasted until about 11 a.m. because of backups of about five miles on the Maryland side and six to eight miles on the Virginia side of the span. Highway officials said motorists who slowed their cars to look at the crash site contributed to the traffic jams.

Police said traffic movement was much better during the evening rush. A one-mile backup was reported at 5 p.m., but traffic was moving freely more than an hour later.