Five firefighters were injured and thousands of federal workers were sent home yesterday after a two-alarm electrical fire and explosion ripped through the attic of the U.S. Customs Service headquarters in Northwest Washington.

Traffic was backed up to the exit ramp of the 14th Street bridge, one of the main arteries into the city from Northern Virginia, as more than 30 fire trucks and other emergency vehicles clogged Constitution Avenue NW between 12th and 14th streets for almost three hours.

D.C. fire department spokesman Peter C. Woolfolk said the 9:42 a.m. fire started in the northwest corner of the U.S. Customs Service headquarters, a colossal eight-story structure at 14th Street and Constitution Avenue, across from the National Museum of American History.

A spokesman for the General Services Administration, which operates federal buildings, said about 2,250 employes from Customs and the Interstate Commerce Commission, which adjoins Customs, were sent home for the day while GSA workers attempted to restore power to the buildings and tested for toxic fumes.

Fire officials said it took more than 80 firefighters about 30 minutes to extinguish the blaze, which caused about $75,000 damage and was started by overloaded electrical circuits in an attic duct.

According to Lt. Ronnie Danner of Rescue Squad 1, the first firefighters on the scene did not know that the duct contained electrical wires, and when they sprayed water into the shaft, "There was a tremendous, thunderous-type explosion, followed by a fireworks display."

Danner said that firefighters were standing on an eight-foot-high wooden cart to spray water onto the fire, which was contained in a duct that ran horizontally through the ceiling.

According to one firefighter, "There was a large flash, and then multiple flashing after that. As soon as we saw the arcing, we jumped. Firefighters could make the Olympics at times like that. You don't just slowly back up."

Deputy Fire Chief Ray Alfred said firefighter Zane Gray of Engine Company 2 "received the bulk of the blast."

Danner said Gray was blown off the cart by the explosion.

"We dragged him out and regrouped about 100 feet down the hall," he said. "There was considerable smoke and we had to find a way to retreat."

Firefighters battled the flames from a distance to keep the fire from spreading, Danner said, and moved in to attack the seat of the blaze after the electricity was shut off about 10 minutes later.

Gray was taken to George Washington University Medical Center and treated for smoke inhalation before being transferred to the Washington Hospital Center, where he was in good condition late yesterday with superficial electrical burns, a spokesman said.

Four other firefighters suffered injuries ranging from smoke inhalation to minor burns and were treated at local hospitals and released, authorities said.

About 1,000 Customs workers were sent home for the day, as were about 1,250 Interstate Commerce Commission employes, according to GSA spokesman Dale Bruce. He said the commerce commission headquarters at 12th Street and Constitution Avenue is serviced by the same power system as Customs. No toxic fumes were found during tests, Bruce said.

Spokesmen for Customs and the commerce commission said workers are expected to report to work as usual today.

Yesterday's unscheduled holiday was greeted by employes as both a welcome surprise and an annoyance, depending on whether they'd had the foresight to take their personal belongings with them when they evacuated their buildings.

After standing on a sun-drenched sidewalk for more than an hour, Clyde Hart, who works in the commerce commission's compliance office, was told he could leave for the rest of the day, but that he could not go back into the building to get his keys until workers had tested the air.

"I might go home and work around the yard, assuming I can get in to get my keys to get into my house," he deadpanned.

"It's getting hot out here," said Cheryl West, a secretary at Customs, as she stood in some shade clutching her pocketbook and waiting to hear if workers would be sent home. The good news was barely out of a spokesman's mouth when she was halfway down the sidewalk, declaring, "I'm going home to chill out."