WASHINGTON -- The U.S. Capitol was briefly evacuated Saturday afternoon when a small aircraft entered restricted airspace, authorities said.
The evacuation was ordered after authorities were unable to immediately contact the pilot, U.S. Capitol Police said.
The aircraft has since left the restricted airspace and the pilot is now in communication, police said. Staff and visitors were being allowed back inside the Capitol.
Also evacuated during the scare was the Library of Congress and the U.S. Supreme Court buildings, the Metropolitan Police said.
Federal Aviation Administration officials said there will be an investigation into why the pilot entered the restricted airspace without authorization.
The incident prompted authorities to institute an AIRCON RED -- air condition red -- alert, police told CNN. The alert is triggered when an aircraft enters restricted airspace in Washington without authorization.
It's not the first time such an alert has been issued.
The U.S. Capitol was briefly evacuated and the White House was temporarily locked down when a single-engine plane wandered into restricted airspace in April 2009.
It was also evacuated in January 2011 when a commercial airliner accidentally entered restricted airspace.