They didn't know what it was -- only that it was loud.

A fiery traffic accident that killed three motorists and seriously injured two others yesterday in the Brandywine section of Prince George's County occurred in front of the home where Brian Saar lives with his girlfriend, Karen Reynolds. The 5 a.m. crash, which involved five vehicles, woke the couple suddenly, and the explosion of a dump truck's fuel tanks and the crack of tires popping like gunshots sent them fleeing to a neighbor's place.

"I thought somebody was coming through the damn house," Saar said. "Boom, boom, boom!"

"Yeah," Reynolds added. "Like terrorists."

Maryland State Police said the crash began when a taxicab traveling north in the left lane of Route 301 swerved left to avoid a Ford Escort that had been abandoned there after breaking down. The cab jumped the curb, careened 35 feet across the grass median strip and collided with a southbound dump truck, police said.

The Peterbilt dump truck then swerved across the median, plowing through small trees and shrubs before it collided with a catering truck in the northbound lanes, police said. A Chevrolet Monte Carlo then rear-ended the catering truck. The dump truck continued in the wrong direction, driving over and flattening a Buick sedan, police said.

The catering truck burst into flames, then the Monte Carlo caught fire and the dump truck exploded, witnesses said. The drivers of the dump truck, the catering truck and the Buick were killed, police said, and the other motorists were seriously injured. Seven hours later, two trucks were hauling away charred and twisted wreckage.

"It was just unbelievable," said Darrold Comber, 42, a truck driver who witnessed part of the crash. "The whole side of 301 was just ablaze."

Last night, police were withholding the names of the three dead motorists until they could be positively identified. The cabdriver, Margaret B. Eyombo, 53, of Waldorf, and the driver of the Monte Carlo, James A. Long Sr., 54, of Hughesville, were taken by helicopter to Prince George's Hospital Center with injuries authorities said were not life-threatening.

When motorist Matthew Jacobs, 29, happened upon the scene, swerving to miss the taxi, the catering truck was in flames and shards of debris were still bouncing along Route 301. He called 911 on his cell phone at 4:59 a.m. He said the first firetrucks arrived 20 minutes later. Several people, including neighbors along the highway, reported that the catering truck's propane tanks exploded. They said they heard the trapped driver's screams for help as the fire burned around him.

"I couldn't approach him. The fire was too hot," Jacobs said. "He died 20 feet in front of me."

Jacobs and others said the fire spread from the catering truck to the Monte Carlo, then to the dump truck, which was mangled on the shoulder of the road on top of the Buick. Jacobs said he saw a "river of flame" crawling up the left side of the dump truck, which ignited the two saddle tanks of diesel fuel in quick succession. The leaping flames melted the fiberglass cab of the dump truck to a charred nub.

Police said they found and interviewed the driver of the abandoned car, whom they declined to identify. They said he told them that the car broke down and that he walked to a gas station to get help.

The investigation is continuing, and no charges had been filed, police said.

"We have a very horrific situation with this crash, and we have to do a detailed investigation," said state police Cpl. Rob Moroney, standing on a highway slick with oil, strewn with broken glass and metal and covered with flame-retardant foam.

"When you get up in the morning and go to work, you can thank your lucky stars when you get home," Moroney said. "You never know what's going to happen out here."

Prince George's firefighters extinguished the flames, which blackened a grassy ditch along the shoulder. After several hours, firefighters cut through the top of the Buick to pull out the remains of the driver.

For miles north and south of the crash site, traffic was at a near standstill. For most of the morning, one lane of northbound traffic was diverted to the southbound lanes, while southbound traffic, closed for six hours, was funneled onto other roads. By noon, traffic was trickling along the northbound and southbound lanes.

"This is the worst crash I've ever seen," said Ronnie Cooke, 49, who works at the Cooke's Shell Station near the crash scene and helped tow the wreckage away.

At a Citgo station south of the crash site, several frustrated commuters leaned on their cars, watching three packed lanes moving nowhere. By 9 a.m., Gary Sherbine, 63, had been waiting nearly four hours to get to his construction job at Andrews Air Force Base. He had sent his crew back to the motel in Waldorf but was not willing to give up himself.

"I must get to that job today," he said.

Many people at the scene spoke angrily about this stretch of Route 301, a frequently congested highway used by more than 80,000 drivers a day and a common route for commuters to Washington.

"Every day when I go to the mailbox, I wonder if I'm going to get killed," said Francis Reidy, who owns a heating and air-conditioning business near the crash site. He said a car came to a stop on the front lawn of his business after a crash a couple of months ago.

"This road is completely inadequate for the traffic flow," Reidy said. "It's extremely dangerous."

Staff writer Jamie Stockwell contributed to this report.

"It was just unbelievable," said Darrold Comber, 42, a trucker who saw part of the crash. "The whole side of [Route] 301 was just ablaze."