The fire was quickly extinguished, but smoke filled parts of the building, adjacent to the White House, fire officials said. The building was largely unoccupied and no one was injured.
However, department spokesman Dan Nelson said early Tuesday the building would be closed Tuesday and only essential employees were expected to report for work.
Fire department spokesman Alan Etter said investigators were looking into whether a faulty transformer was to blame for the fire in the building's basement.
There was some concern for fish and other marine life kept in the National Aquarium which is in the basement. Power needed to run pumps for the aquarium had to be shut off, said Etter.
Firemen and aquarium officials were looking for generators to restart the tank pumps.
