Shouldn't there have been a plan for that contingency already in place? And what evidence do you have that there isn't such a plan in place? And maybe it was even put into action. One would hope that our national defense forces would have considered this possibility, since even the Columbine killers had thought of the same plan, hijack a plane and crash it into NYC.
Tell me your politically acceptable plan to prevent the next really big terrorist attack.
Please bear in mind that preventive measures are generally known to the public, and terrorists will know them and attempt to plan around them. They might even take advantage of your policies and past behavior.
So knock yourself out. Predict the future.