Staff were briefly evacuated from the White House in Washington on Thursday after an aircraft was thought to have strayed close to it, US officials say.
But a US aviation official later said the alarm was caused by a "blip" on a radar screen, rather than a plane.
Two F-16 fighters were scrambled from Andrews Air Force Base whose pilots confirmed that there was no threat.
The emergency is now said to be over and staff have been told it is safe to return to work.
US President George W Bush, who has his office and residence at the White House, is currently on a state visit to the UK.
Federal Aviation Administration spokesman William Shumann said the emergency had been caused by a "false radar target", and fighters from the US air defence command had found no trace of a plane.
Flocks of birds or atmospheric disturbances might have caused the blip, he said.
"It's one of those electronic gremlins that pops up, but there was no aircraft there," he said.
The Secret Service, the US president's security agency, said that while some people were removed from the building there was no official evacuation order.
They were allowed to return soon afterwards.
Some reports say that US Vice-President Dick Cheney was whisked away from the White House in a motorcade.
The incident follows a similar alert 10 days ago when two F-15 fighter jets were scrambled to intercept a plane.