From
theHill.com:
The Senate Hart and Dirksen Office Buildings were evacuated shortly after 1 p.m. on Wednesday.
There are at least four fire trucks and an ambulance at the scene as hundreds of aides left the buildings. Senate staffers told The Hill that this was not a planned drill, though it is unclear why fire and rescue personnel were called.
Evacuating hart & dirksen due to confirmed fire & audible alarm. U.S. Capitol Police spokeswoman Sergeant Kimberly Schneider, said in an e-mail.
More on this as it develops...