WASHINGTON, Nov 20: A “blip” on a radar screen, rather than a plane, sent scores of White House staff and tourists fleeing the executive mansion on Thursday in fear of another Sept. 11-style attack on the US capital.

Two Air Force F-16 fighters were scrambled to secure the air space over the White House and Vice President Dick Cheney was whisked away in a motorcade to an undisclosed site.

Secret Service agents, some with shotguns drawn, hustled other senior staff members and visiting school children away from the White House and adjacent offices before the incident was determined to be a false alarm.

US President George Bush was in Britain at the time, and his staff returned to work in the White House complex shortly after the evacuation ended.

US stocks slumped during the brief security scare, which came on the same day that two blasts in Istanbul, Turkey, killed at least 25 people, wrecking the British consulate and the headquarters of a bank.

“There was never a plane. It was a blip on one radar,” said Federal Aviation Administration spokeswoman Rebecca Trexler.

The Secret Service said it followed normal procedures by initiating an evacuation at 920am EST (1420 GMT) after receiving reports of a “violation of the flight restricted zone” around the White House. Air space over the nation’s capital has been restricted for years but the off-limits area was expanded after the Sept 11 attacks.

As the incident unfolded, some Secret Service guards brandishing shotguns were on the sidewalk in front, cautioning people to stay away from the White House. One shouted “Get out of here” to a cluster of people gathered across Pennsylvania Avenue from the White House.

NORAD BUT NO THREAT: The North American Aerospace Defense Command, or NORAD, said two Air Force F-16 fighters were scrambled from Andrews Air Force Base near the capital “in response to an indication of a Washington restricted air space violator,” but found nothing in the prohibited zone.

“The fighters determined that there was no threat but continued to patrol the area,” said Air Force Maj. Eric Butterbaugh, a NORAD spokesman.

The Secret Service said the evacuation was ended after “the airspace violation was determined to be a radar anomaly.”

It was the latest in a series of security scares for the White House.

On Nov 10, a small plane caused a brief security alert before military jets intercepted it and steered it away.

On Nov 1, a woman in Mississippi rammed a car carrying children into a building where Mr Bush was campaigning, drawing a swarm of police in her wake before being dragged away at gunpoint. Officials said the woman had personal problems and never targeted the president.

On Oct 9, police in New Hampshire took a woman into custody after she drove her car to within 800 feet (240 metres) from Air Force One, the president’s plane. The woman told police that she believed she had radiation poisoning and was going to the airport to somehow stop it.

White House spokesman Trent Duffy said Mr Bush’s staff was unfazed by the latest scare. “The mood is fine. ... We returned quickly to our desks.”—Reuters