Washington ? Pilots have flown through the prohibited airspace protecting the White House at least 94 times during the past decade, illustrating the challenges of thwarting a terrorist airstrike on the nation’s capital.

Even with military jets patrolling the skies, four commercial airliners and a medical helicopter have crossed into Washington’s no-fly zone since the Sept. 11 suicide hijackings, Federal Aviation Administration officials say. The latest was on Monday.

In most cases, pilots who violated the airspace protecting the White House, vice presidential mansion and Capitol have gotten penalties less severe than a parking ticket, an Associated Press review of FAA enforcement records found.

Just a month before the September hijackings, a Mesa Airlines flight strayed into prohibited airspace. By November, the matter was closed with a warning letter to the pilot common for most cases.

Security experts say violations of the Washington airspace highlight a key reality in the fight against terrorism planes that veer into the zone can crash into government installations within seconds.

“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 recognized there was 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 must complete authentication procedures, including providing passwords.

About three dozen planes approaching Reagan National Airport have been turned away since Sept. 11 because they didn’t complete the verification process, officials said.

Planes that violate the prohibited zone are quickly warned by the flight tower to correct course, and the Secret Service is alerted. Nearly all pilots comply immediately, officials said.

Military planes that patrol the capital skies are permitted to force such planes to land or, as a last resort, shoot them down if pilots don’t respond.

None of the five planes that flew into the protected space since Sept. 11 have required such action, officials said.