The White House and the US Congress building were evacuated briefly when a light aircraft entered the no-fly zone over Washington DC.
The alert sparked a flurry of emergency activity
People were seen running out of the Capitol after the alert was given just before noon (1600 GMT) on Wednesday.
Staff in the Treasury and the Supreme Court were also moved to safety. They were allowed to return shortly after.
The plane was within three miles (5km) of the White House when it was forced to turn around.
President George W Bush was not in the White House at the time.
The aircraft, a two-seater Cessna 150 with a top speed of about 100mph (160km/h) appeared on radar at 1128.
Air traffic control tried to contact the pilot when the plane was 25 miles (40km) west of Capitol Hill.
At 1155 the customs department launched at least one Black Hawk helicopter to intercept the aircraft, which was registered to an air club in Pennsylvania.
Five minutes later, two military F-16s jets fired four warning flares when the Cessna's pilot did not respond to radio calls.
The light aircraft was within three miles of the White House when it started to move away, escorted by fighter jets.
By 1237 the aircraft had landed in neighbouring Maryland.
A pilot and a student pilot aboard the Cessna - identified as Jim Sheaffer and Troy Martin - were questioned by the FBI and the Secret Service.
The two men were later released without charges, reports said.
Their intrusion appeared to have been accidental, as they were flying to an air show.
The colour-coded terror alert was raised from yellow to orange to red and then lowered when the plane was diverted, the White House said.
As the alarm was raised, 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.
Senator Richard Shelby was on the Senate floor when police told him to evacuate the building.
"They said get out of here, so I ran," he told the AP news agency. "There's no joking about this kind of stuff."
President Bush was away from the White House at the time, riding his bicycle.
But Vice-President Dick Cheney - who was present - was taken to a safer location.
Airspace over Washington DC is restricted in a radius of about 30 miles (48km) from the US Capitol.
There are plans to introduce a new laser system within days. It will use red and green lights to warn pilots if they enter the restricted area without the appropriate identification.