North American Aerospace Defense Command (NORAD) on Tuesday is conducting interception and identification exercises over Washington. 

Low-flying helicopters and military aircraft are scheduled over Washington as well as Charlottesville, Va., Frederick, Md., and Mechanicsville, Va. 

NORAD had regularly conducted exercises simulating hijackings and airspace restriction violations since the Sept. 11, 2001 attacks. It has responded to more than 5,000 potential threats since 2001.

Planes occasionally enter the restricted airspace over Washington by accident. NORAD scrambled two F-16 jets in June after a small aircraft wandered into restricted airspace. 

The event caused the brief evacuation of the Capitol after air traffic controllers were unable to get in touch with the pilot. The plane was eventually escorted to an airport in North Carolina.