A Georgetown street collapsed Wednesday night because steel I-beams at a construction pit suddenly gave way, authorities said yesterday, though they added that they do not yet know what caused the beams to fail.

Traffic disruptions continued yesterday around the site of the collapse, on tiny Bank Street NW, as trucks fought through traffic, carrying dirt to shore up the street. Though a wider collapse had been feared at first, officials said there was no further danger.

"At this point, we feel that people shouldn't be concerned," said Chris Bender, a spokesman for the District's office of planning and economic development. "We can feel confident saying that the rest of Georgetown is safe."

By yesterday's evening rush, five of six lanes of traffic were open on a three-block stretch of M Street. One lane was expected to remain closed this morning, officials said, and they warned that trucks backing out of Bank Street might slow commuters.

The collapse happened about 7 p.m. Wednesday just off M Street NW, on a one-block side street barely wider than an alley. The construction pit at Bank and M streets is at a site that used to hold a liquor store and the Little Tavern. Developer Eastbank Inc. is planning to build a two-story retail complex with a parking garage underneath.

Excavation began last summer, the developer said, and was near completion when the accident occurred.

The sides of the construction pit were held up by a retaining wall, with horizontal slats secured onto I-beams sunk into the dirt.

Bender said four beams on the eastern side of the pit fell in, causing a portion of the wall and part of the neighboring street to tumble.

Bender said there were reports of a water main break in the area before the collapse, leading some to suspect that the soil around the pit could have become sodden and pliant.

But the D.C. Water and Sewer Authority rejected that theory, with a spokeswoman saying that there were no leaky pipes in the area beforehand, though a six-inch water main was broken by the collapse.

The city has issued a stop-work order for the construction site, except for work related to shoring up the collapse, Bender said. This morning, he said, city inspectors and transportation officials plan to meet with construction officials to try to determine why the beams gave way.

Officials from Eastbank developers and from Kadcon Corp., the construction contractor working on the site, said they were unaware of any previous instability or earlier worries about a collapse.

City officials said yesterday that they were unable to even guess at when Bank Street might reopen or when the construction project would be completed.

No one was hurt in the collapse, which apparently was first detected by D.C. police on patrol. Officer Alton Bryant said yesterday that he noticed a new crack about five inches wide in the middle of the street. The pavement seemed uneven, Bryant said, sitting slightly lower on one side of the crack.

"As I looked, it was getting further and further and further" apart, Bryant said yesterday, "like it was an earthquake." He and another officer began evacuating nearby homes and a Kinko's copy store, telling patrons there was no time to retrieve their cars from an underground garage.

Then, without making a sound, one side of Bank Street slid into the pit, Bryant said.

Residents of a six-unit apartment building still were barred from returning yesterday, and police said they might be displaced for as long as a week.

Engineers examine the area where Bank Street NW collapsed. A police officer said the street split "like it was an earthquake."A developer plans to put a two-story retail complex atop a parking garage at the site at M and Bank streets NW where the collapse occurred Wednesday night.