Several governmental buildings in Washington, D.C., were evacuated Thursday morning due to a suspicious vehicle and what Capitol Police call "an active bomb threat investigation."

       Law enforcement negotiators are working to make contact with the person in the vehicle, law enforcement sources told ABC News.

       The Cannon House Office Building, a congressional office building, was evacuated via underground routes. The Library of Congress and Supreme Court building were also evacuated. Both the Supreme Court and Congress are on recess.

       Messages have been sent to congressional staffers asking that they "remain calm and relocate to Longworth House Office Building using the underground tunnels."

       Senate staff were asked in a message to "remain clear of the police activity" and to "please move indoors" if they were outside on Capitol grounds.

       ABC News

       Police activity is pictured near the Cannon House Office Building, which is being evacuated due to a suspicious vehicle at 100 block of 1st St., SE, according to a law enforcement sources. The Library of Congress is also being evacuated.

       ABC News

       Police activity is pictured near the Cannon House Office Building, which is being evacuated due to a suspicious vehicle at 100 block of 1st St., SE, according to a law enforcement sources. The Library of Congress is also being evacuated.

       The FBI said it's responding. The White House is monitoring the situation and is receiving updates from law enforcement, according to an administration official.

       This is a developing story. Please check back for updates.