by 
No one knows why a small private plane flew into restricted airspace in the nation’s capital on Saturday, or who was responsible; but it caused the evacuation of the US Capital and a few surrounding office buildings.
The buildings were ordered evacuated by police when the pilot of the plane did not immediately communicate after it entered into restricted airspace, at about 1:30pm. When the pilot did begin to talk, the evacuation was lifted, and people returned the buildings at about 2:00pm.
Apparently planes fly into restricted airspace regularly in Washington DC, but they usually do not lead to building evacuations. The trigger was most likely the lack of communication on the part of the pilot.