President Bush was rushed to a secure underground White House bunker and Vice President Dick Cheney (search) was whisked outside the compound Wednesday because of a "radar anomaly" — perhaps a flock of birds or pocket of rain — that was mistaken for a plane flying in restricted airspace.

The late morning scare was determined within minutes to be a false alarm, and business quickly returned to normal. Later in the day, security officials sent a robotic device to investigate an unattended package left along Pennsylvania Avenue (search) in front of the White House.

There have been similar alarms before, sparked by a blip on a radar screen that looks like an aircraft venturing into the area around the White House that is off-limits to aircraft.

In November 2003, the White House was briefly evacuated while Air Force fighter jets were scrambled to investigate a tripped radar alert that also triggered fears — also groundless — of an airspace violation. Bush was in Britain at the time.

This time, though, Bush was in the Oval Office when radar picked up something. Helicopters were sent to check it out and found there was no errant aircraft, said Brian Roehrkasse, a Homeland Security Department (search) spokesman.

Before that could be confirmed, though, the Secret Service (search) leapt into action.

They moved the president and vice president out of danger. Officers toting shotguns raced around the compound taking up positions. A surface-to-air missile battery recently installed on the roof of a nearby building was raised to fire position. Some White House staff members were evacuated from the West Wing. Tour groups were hustled out of the executive mansion and a park across the street from the White House was cleared.

Some parts of the compound, such as the area where the press is housed, were not notified of the threat or moved.

There are a few security incidents at the White House each year, and many of those involve the threat of a plane in restricted airspace. At least a half-dozen such threats have occurred during Bush's presidency, either at the White House, Camp David, or events outside Washington.

In 1994, when Bill Clinton was president, a Maryland man stole a light plane and crashed it into the south side of the presidential mansion in the middle of the night, killing himself but injuring no one else. In 1974, an Army private landed a stolen helicopter on the South Lawn. He suffered shotgun wounds and was taken away for psychiatric examination.

There are other types of attempted security breaches as well, including suspicious packages and "gate-jumpers" who try to force their way onto the White House grounds. In February 2001, a man fired several shots outside the White House fence and was shot in the leg by authorities. In 1976, a man tried unsuccessfully to ram his pickup truck through the White House gate.

Bush went to the bunker — a super-secure, super-secret facility built in case of emergency for the president, his family and a small number of staff deemed essential to running the government — for the first time on the night of attacks of Sept. 11, 2001. After flying from the Florida classroom where he learned of the attacks to Barksdale Air Force Base in Louisiana and then to Offutt Air Force Base in Nebraska, Bush returned to Washington only to be awakened by Secret Service agents who hurried him and his wife, Laura, underground — also because of a false alarm about an unidentified plane.

Since then, Bush has gone to the bunker, White House spokesman Scott McClellan said. He provided no details.

On Wednesday, Bush was in the shelter for just a short time, McClellan said.

Cheney's movement out of the White House and to a secure undisclosed location couldn't have taken him far. His limousine was seen re-entering the compound gates immediately after the situation was resolved.

It wasn't known what caused the radar anomaly, said Federal Aviation Administration spokesman Greg Marting.

"We appreciate the precautionary steps that the Secret Service took," McClellan said.