Planes frequently have crossed into protected airspace since Sept. 11

WASHINGTON (AP) -- Despite military patrols and tighter security, pilots have intruded into America's protected airspace at least 567 times since Sept. 11, highlighting the continued challenges of thwarting a terrorist airstrike.

In each case, a pilot wrongly flew into one of the country's six prohibited flight zones, where no planes are allowed, or into one of many restricted zones where air traffic is limited because of sensitive military or nuclear operations or special events.

The post-Sept. 11 incidents include four commercial jetliners and one medical helicopter that flew into the forbidden airspace protecting the White House, Capitol and vice presidential mansion in the nation's capital, officials said. The most recent incursion occurred this week.

"Practically speaking, by the time a violation is discovered, it is too late to do anything to prevent a crash into the White House," former FAA security chief Billie H. Vincent said.

FAA Deputy Administrator Monte R. Belger said Thursday the agency recognizes there's little time to react once planes penetrate the safety zone and so the government has imposed numerous other precautions to ensure planes with ill intent don't get close.

"The restricted area is kind of the last line of defense," Belger said. "The additional on-the-ground security procedures and in-flight protocols put in place give us a much higher level of confidence."

Borders have been tightened; pilots, flight crews and passengers are screened to weed out possible terrorists, and planes approaching Washington or other prohibited zones must complete authentication procedures, including providing passwords.