The familiar boom, flames and plumes of smoke that herald manhole explosions returned to Georgetown yesterday when an underground blast sent at least one manhole cover aloft, filled one of Washington's premier shopping streets with the sulfuric odor of natural gas and created a slushy gridlock that delayed some evening commutes by hours.

"It sounded like a gunshot. Or like a car drove into the building," said David Frandono, assistant manager at the J. Paul's restaurant, which at 3218 M St. was in front of the explosion's epicenter. "Then we looked at the street and saw manhole covers like 10 or 15 feet in the air."

Immediately after the 4:40 p.m. blast, Frandono and about 15 customers in his restaurant smelled natural gas. D.C. fire officials found strong readings of gas in the air along the southern side of that block and ordered the evacuation of three businesses, including J. Paul's, said department spokesman Alan Etter. Etter also said that he believed one manhole cover dislodged and that another emitted smoke but stayed intact.

It was unclear whether the gas leak or the explosion came first. Washington Gas workers drilled holes into the street's asphalt to vent the gas, but no utilities had to be turned off as a result of the blast, said Tim Sargeant, a spokesman for Washington Gas.

Late last night, Washington Gas officials found damage to a three-fourths-inch ventilation pipe, which they said prompted the leak, about 20 to 30 feet from the explosion. "What we still don't know is what may have served as an ignition source," Sargeant said.

With the threat of a natural gas explosion, police closed M Street from Wisconsin Avenue to the Key Bridge, overwhelming many of Georgetown's narrow roads -- some of them still unplowed. Police also stopped incoming traffic on the bridge. Two-way traffic on M Street wasn't restored until about 9:30 p.m.

"We still have a couple of streets up here that need work or haven't been plowed yet," said Dan Tangherlini, the city's transportation director. "We needed this like a hole in the head."

D.C. police used their traffic cameras to try to route traffic to less congested arteries. Still, backups stretched into the heart of downtown, along M and K streets and other westbound commuter routes. Traffic was bumper to bumper well past 8 p.m., with cars slipping, sliding and spinning their tires on the snow-clogged pavement.

Officials expect traffic along M Street to be unimpeded this morning.

"It was the mother of all traffic jams," said Carol Joynt, owner of Nathan's, a tavern and restaurant on M Street, who lives in Georgetown and is a veteran of many manhole explosions. "I've been through enough of these that my heart rate doesn't speed up anymore when it happens."

Underground explosions had been quietly popping throughout the District for years, but they made headlines in Georgetown on Feb. 18, 2000, when a blast sent three covers about 30 feet into the air, shattered storefronts and publicly revealed a deteriorating infrastructure in the historic neighborhood. The Georgetown substructure -- parts of which were created in the 1800s and never upgraded -- was most vulnerable during the winter, when road salt eroded protective casings.

The cluster of blasts that year prompted a massive overhaul of Georgetown's underground systems, a $40 million project involving electrical, water, gas and communications lines that began in October 2001 and is scheduled to end in the summer of 2005. Yesterday's blast and gas leak occurred on the same block where Washington Gas completed a 200-foot section of work on the south side of the street two weeks ago, officials said.

The manhole explosion came as a surprise to Ryan Furr, manager of the recently opened Pizzeria Paradiso, which shut early because its section of M Street had been closed.

"I thought they had fixed" the manhole problems, Furr said, frustrated by an estimated $1,200 in lost revenue last night.

At the Deep Dish Records operational office on M Street, accountant Niki Tsakonas wasn't expecting the loud boom that Georgetown knows so well.

"Our building even trembled. . . . I thought there was another terrorist attack. I yelled," Tsakonas said. "Then somebody said, 'Oh, it was probably just a manhole.' "

Staff writers Carol Morello, David A. Fahrenthold and Allan Lengel contributed to this report.

Firefighters and Washington Gas workers investigate the site of the 4:40 p.m. manhole explosion in the 3200 block of M Street. Traffic was bumper to bumper well past 8 p.m.