U.S. Capitol Police on Thursday are investigating what they called “an active bomb threat” from a truck parked near the Library of Congress, and ordered the evacuation of the library a nearby House office building.

The Capitol Police Twitter feed warned the public to “please stay away from this area.

In addition to Capitol police, Washington police were at the scene, some of whom had gas masks.

People at the Cannon House Office Building received an alert telling them to leave that building and relocate to the Longworth House Office Building.

The FBI Washington Field Office’s National Capital Response responded to the scene, as did agents from the Bureau of Alcohol, Tobacco and Firearms.

Congress is currently in recess.

The White House was monitoring the situation, and getting updates from law enforcement.