A fire and EMS spokesman said the blaze started at about 11 a.m. ET Saturday in the ductwork on the 7th floor as crews were working on the building. Workers were able to put the fire out before firefighters arrived, but not before three people suffered burns.
Authorities said one person suffered life-threatening injuries and two others had serious, but non-life-threatening injuries. All three were transported to Washington Hospital Center.
There were few federal employees in the building because of the holiday weekend.
The Associated Press contributed to this report.