Police have evacuated the US Capitol and the visitors' centre.
Capitol police provided no explanation for the move, simply saying it was an emergency.
In a message to congressional offices, police said "an evacuation has been ordered for US Capitol Visitor Centre due to an audible alarm. Remain calm and move in a safe manner to the exits".
Hundreds of tourists were forced out of the building on to the East Lawn between the Capitol and the Library of Congress.
Fink Arthur, a 28-year-old tourist from Freiborg, Germany, said he was with a tour guide and was initially told it was probably just a test but then the guide led them out of the building.