Law enforcement officials surrounded a man with possible explosives sitting in a vehicle near the US Capitol on Thursday, as nearby buildings were evacuated and emergency vehicles rushed to the scene about a mile from the White House.
"This is an active bomb threat investigation," the US Capitol Police said on Twitter.
The man was in a vehicle outside the Library of Congress, across the street from the Capitol, a US law enforcement source said.
The law enforcement source said the presence of explosives had not been confirmed.
But people in Congress were notified that the vehicle's occupant had said that he possessed a bomb and that authorities were crafting their responses with that threat in mind, an official told Reuters.
Several nearby buildings were evacuated, including the US Supreme Court. People in the Madison office building were told to bar themselves in their offices. A nearby subway station was closed.
Police blocked off roads surrounding the Capitol complex as fire and rescue trucks headed to the area. The Bureau of Alcohol, Tobacco, Firearms and Explosives said it was sending a bomb technician to support police. The Federal Bureau of Investigation also responded.
The ordinarily crowded Capitol Hill area was relatively deserted, with the US House of Representatives and Senate out of session.
"The USCP is responding to a suspicious vehicle near the Library of Congress," the Capitol Police wrote on Twitter. "Please stay away from this area."