Bradylama said:
But you're still not getting the point. IDs don't identify intent. How would they have known who the terrorists were on the plane so soon after the crash simply based on ID? Racial profiling? Conjecture? They weren't exactly known terrorists, here. They wouldn't have been able to blend in and become citizens otherwise.
What if it was a white person that hijacked a plane and crashed it? An IRA sympathizer? A crazed nut? How would we know who crashed the plane then?
Based purely, keep in mind, on ID.
A database.
You know each of the people on the plane from ID.
A fake ID has information on it which is fake.
Cross check the information kept from the IDs as much as possible. If an ID says they lived such and such a place, then ring them up and ask if the person actually lives there. The vast majority of proper IDs will be able to be verified the fake one wont be, if it's a fake it'd be a good assumption that the person carrying the fake ID is the terrorist, you now do not know who that person is, but you do have a starting point and can see if the fake ID was used elsewhere. You create a profile of the movements and actions of the person with absolutely no information about the person themselves. From this you can get packets of information quickly.
If it's a proper ID then you can start data mining and cross checking and come up with a list of most likely candidates to start work on. You can mark people from different dangerous countries or ones known for sympathies or contact with terrorists, even better, if they piloted the plane, simply run up a check on each ID against pilots liscence or against flight schools.
If it's a lone nut with proper ID then it's no where near as effective, there are no accomplices to stop and they're dead. However you may get information as to how they did it, where they got materials and when.
If it a first or only strike it wont prevent it except one in a thousand cases, but it means that afterwards you can go after their mates, contacts and equipment quickly which in turn can save lives.
It can also be used as a method to trace, detain and sieze under certain circustances.
It gives you the edge, it may be the only information you have, and it may lead you to a terrorist cell, plans or equipment.
Oh and security isn't the only reason for IDing people before boarding a plane.