Police are investigating an "active bomb threat" near the Congress building in Washington DC.
US Capitol Police said they were negotiating with the driver of a suspicious vehicle near the Library of Congress, across the street from the Capitol building and Supreme Court.
The motives of the driver remain unknown, and he was livestreaming from the vehicle, they said.
Several office buildings have been evacuated.
Congress is on recess, but some staff are working in the Capitol.
The man parked the vehicle on a pavement and told an officer who approached him that he had a bomb, while holding what appeared to be a detonator, Capitol Police Chief J Thomas Manger told a news conference.
"My negotiators are hard at work trying to have a peaceful resolution to this incident," he said.
Some members of staff on Capitol Hill were originally told to remain in place and shelter in offices.