Temporary office space was being sought yesterday for employes displaced in the fire at the U.S. Postal Service Headquarters at L'Enfant Plaza as investigators continued to search for the cause of the multimillion-dollar blaze.

A second fire broke out in the building yesterday morning when smouldering file cabinets on the ninth floor -- where the first fire started -- briefly erupted in flames, but firefighters quickly extinguished the blaze, D.C. fire officials said.

A spokeswoman said the 2,800 Postal Service employes who work in the building were told not to report to work today, unless instructed otherwise, and would be paid in full for time missed as a result of the fire.

"Temporary office space is being looked into," said David McLean, the director of the Postal Service's news division. "Almost for sure some people are going to have to work in temporary space somewhere. There's just too much damage."

The fire, which started late Monday night, caused an estimated $100 million damage to the 11-story building, according to D.C. fire officials.

Postal officials, who said the building was purchased in 1972 for $23 million, said the fire department estimate might be high and their own "damage assessment team" was touring the building to assess the damage, most of which was caused by smoke and water.

District fire officials had no clear itemization of their damage estimate, but one said that "every desk in the building" had a computer on it. In addition, Mary Jane McKinven, a spokeswoman for the Public Broadcasting Service, which also has its headquarters in the building, said there was a "great deal of broadcasting transmission equipment" in the basement, where large amounts of water have accumulated.

Twenty-five firefighters were overcome by smoke inhalation and exhaustion while fighting the blaze. Yesterday, firefighters were critical of the 30-minute air tanks they used during the fire, claiming that money to purchase 60-minute tanks has been in the fire department budget for two years but was allocated instead to overtime pay.

"By the time they got up to the fire floor, their pack alarms rang and it was time to leave," one firefighter said. "If they had the hour tanks, they could have attacked the fire and made headway. But they were running out of air."

Fire inspectors, arson investigators and members of a team from the Treasury Department's Bureau of Alcohol, Tobacco and Firearms combed the debris on the ninth floor searching for clues to the cause of the fire, officials said.

"It's a mess," said Charles Thomson, head of the ATF team of investigators. "We're just starting to clean away the fire-damaged area. . . . We're at the shovel stage."