Part of a downtown Washington excavation site caved in with an explosive roar and a blaze of sparks last night, swallowing up sidewalks and construction trailers and raising fears that victims could be trapped beneath tons of debris.

The sudden, unexplained collapse at 14th and H streets snarled Metrorail service and street traffic in the area last night, aroused concern for the stability of a nearby building and threatened to cause surface tieups this morning.

Shortly before 2 a.m., Metro spokeswoman Marilyn Dicus said clearance had been received from the fire department to provide rail service in the area this morning, although the 14th and I Street entrance to the McPherson Square station may be closed.

Construction workers told authorities that all of their coworkers appeared to be accounted for at the site, but fire officials said it was possible that pedestrians had been hurled into the pit as a sidewalk along 14th Street collapsed. "That sidewalk is usually heavily traveled," said D.C. Fire Chief Ray Alfred.

Early this morning, Alfred said rescuers had decided not to enter the pit until dawn. Authorities said they were considering sending specially trained dogs into the excavation first in a search for possible victims.

Alfred said 14th Street between I Street and New York Avenue would remain closed today until further notice. He said H Street would be closed from 15th Street to New York Avenue.

Shortly after the collapse, Metro's Blue and Orange lines were shut down between the Farragut West and Metro Center stations because of concern about vibrations caused by trains.

The cave-in, which occurred about 8:30 p.m., filled the eastern and northern edges of the five-story-deep excavation with debris, including three trailers, heaps of splintered timbers and a section of a construction crane. Cracks opened in the pavement of H Street NW.

The site is the same one where a parking garage collapsed last New Year's Eve, killing a worker who was helping to demolish the structure. A 12-story office building is to be constructed there.

Specially equipped cave-in rescue units from suburban fire departments came to the scene to assist 150 District firefighters. A "thermal imager," or heat sensor, used by one rescuer gave a positive indication, a sign that a victim might have been found under the rubble. But Alfred said the reading had not been confirmed.

Witnesses said the collapse occurred without warning, producing a roar and a series of flashes that illuminated nearby buildings. Some said it sounded as if there had been an explosion.

"I saw the whole sidewalk fall in, the lamp post, the tree -- everything just fell in," said Arnell Morris, who was crossing 14th Street at the time. "It just went in."

Cab driver Jim Lampkin said he was heading south on 14th Street from I Street when the collapse occurred. "The walls just caved in," he said. "The street lights, everything went in. It was like fireworks."

Alfred who was nearby and who reached the scene within minutes said he saw sparks at the scene as he approached and that "it was lighting up quite a bit."

Witnesses said the sidewalk along 14th Street had been boarded over to create a protective tunnel for pedestrians.

Construction trailers were mounted atop the walkway.

Staff writer Carlos Sanchez contributed to this report.