(Xinhua)
Updated: 2008-03-13 07:27

WASHINGTON -- A small plane unintentionally intruded the restricted airspace over Washington, D.C., Wednesday, putting the Capitol building on alert but posing no imminent threat.

The US Capitol building is seen in Washington in this January 28, 2008 file photo. A small plane unintentionally intruded the restricted airspace over Washington, D.C., Wednesday, putting the Capitol building on alert but posing no imminent threat. [Agencies]

A short time later, Capitol Hill police gave the all-clear and said the aircraft landed outside the restricted zone, according to wire and TV reports.

There was no evacuation in the Capitol building and "things are back to normal," police said.

The North American Aerospace Defense Command (NORAD) said the plane, a small Cessna, has landed at a regional airport in Leesburg, Virginia, after the intrusion.

The US Coast Guard, which monitors the airspace above the district, scrambled helicopters as a precaution.

The US military also dispatched F-16 fighters from Andrews Air Force Base as a precautionary measure.

Such airspace violations are not uncommon.

There are two concentric restricted zones over Washington, D.C., the outermost called the Air Defense Identification Zone or ADIZ.

Many pilots unintentionally violate the ADIZ as they move out of the zone's outer fringes.

The ADIZ was reshaped on August 30, 2007, from a "Mickey Mouse" shape to a circular shape to help reduce accidental incursions.

The innermost restricted airspace is the Flight Restricted Zone, or FRZ -- a smaller and more restrictive zone. With a few exceptions, only scheduled airlines and governmental flights are allowed in the FRZ.

The Federal Aviation Administration added four air traffic controller positions last summer to help pilots transit the airspace.

Pilots had complained about having trouble getting permission to enter the airspace, forcing them to circle outside, which sometimes caused safety issues.