US Capitol Police are responding to reports of a possible explosive device in a truck near the Library of Congress, law enforcement officials have said.
People in and around the Cannon House Office Building have been evacuated after concerns over the suspicious vehicle, which is located near the Capitol and Supreme Court.
Police say investigators on the scene were working to determine whether the device was an operable explosive and whether the man in the truck was holding a detonator.
“This is an active bomb threat investigation,” police said on Twitter.
Congressional staff offices nearby have also been evacuated, with people told to remain calm and use the underground tunnels to leave if necessary.
The area was blocked off by police cars and barricades.
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are also involved in the investigation.
Congress is on break this week.