Security officials in several other government buildings, including the Treasury Department and the US Supreme Court, ordered people to safer locations.

President Bush was away from the White House, biking at a Patuxent Wildlife Research Center in Beltsville, Md. Congressional leaders were moved from the Capitol.

A small Cessna aircraft breached the security zone over Washington, several law enforcement officials said, prompting alerts across the city. The plane was approached by a fighter aircraft and veered away, according to a Federal Aviation Administration official speaking on condition of anonymity.

But in the meantime, armed security officers raced through the Capitol shouting for people to leave. "This is not a drill," guards shouted as they moved people away from the building.