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 said investigators on the scene were working to determine whether the device was an operable explosive and whether the man in the pickup truck was holding a detonator.
“This is an active bomb threat investigation,” police said on Twitter.
After telling law enforcement he had an explosive, the suspect has been identified by investigators, who said negotiations with him are ongoing.
He has been writing notes and showing them to police from inside the vehicle, according to an official.
The presence of explosives has not been confirmed.
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Earlier, officers noticed the truck had no licence plates and reported a possible bomb threat over police radios.
Most lawmakers were not on Capitol Hill when much of the complex went into lockdown, because the House of Representatives and the Senate are currently on break.
Congressional staff in the Cannon House Office Building have been relocated to the neighbouring Longworth building, with people told to remain calm and use the underground tunnels to leave if necessary.
The Library of Congress’s Jefferson building was also evacuated, while those working in the Madison building nearby were instructed to bar themselves in their offices.
The area was blocked off by police cars and barricades, with fire trucks and ambulances on the scene.
The FBI and the District of Columbia’s Metropolitan Police are also involved in the investigation, with assistance from the Bureau of Alcohol, Tobacco, Firearms and Explosives, who said it was sending a bomb technician to assist police.
Officers said they are “responding to a suspicious vehicle near the Library of Congress,” adding: “Please stay away from this area.”
The FBI and the District of Columbia’s Metropolitan Police are also involved in the investigation, while the Bureau of Alcohol, Tobacco, Firearms and Explosives said it was sending a bomb technician to support police.
The White House said it is monitoring the situation and receiving updates from law enforcement.
The incident comes months after pro-Trump rioters stormed the US Capitol and two pipe bombs were discovered outside the offices of the Republican and Democratic national committees.