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, Reuters reported.
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."
A Republican National Committee spokeswoman said the RNC offices nearby were evacuated as well CNBC reported.
“My negotiators are hard at work trying to have a peaceful resolution to this incident,” the US Capitol Police official told reporters at a briefing.
The suspect has been making anti-government statements, according to law enforcement officials who spoke with NBC News.
Officials said the man claimed to have a propane tank inside the cab of the truck, and also claimed to be holding a detonator. But officers have not been able to look inside the cab.
Two law enforcement officials told NBC that man in the truck is communicating with authorities by writing on a dry-erase board that he had in the vehicle.
Bomb technicians from the FBI and Bureau of Alcohol, Tobacco, Firearms and Explosives were at the scene, in addition to FBI negotiators.
US Capitol Police, who first responded to the incident, warned the public to stay away from the scene.
People at the Cannon House Office Building received an alert telling them to leave that building and relocate to the Longworth House Office Building.