Indianguy is correct. These terrorists are often "sleepers", placed in the target country just waiting for the right moment.
I've heard a lot of people (including on CNN) say that the attack was in response to the U.S. withdrawing from an active role in the middle east, a decision made by President Bush. This is utter crap, as the people who performed this attack had come to the U.S. and were beginning their pilot training no later than the summer of 2000 - well before Bush arrived, and while Clinton was pushing hard in the M.E..
I really feel that this will have to be an all-or nothing campaign. There are essentially 4 options:
1) The U.S. caves in, and abandons all support of Israel and Saudi Arabia. This is of course politically (and morally) out of the question. The targeting of the U.S. by terrorists will stop temporarily, but will be encouraged in the future for other goals, seeing that U.S. has set a precident for giving in to terrorism.
2) The U.S. does nothing. Disappointed at the lack of a response and emboldened by the apparent helplessness of their target, terrorists will escalate to even greater acts of violence.
3) One or several symbolic strikes, such as the previous administration performed against Iraq and Afganistan. This will only piss them off further, gain them new recruits, and lead to even more vicious attacks.
4) All-out destruction of known terrorist strongholds and support sources, with extreme prejudice and determination. Short term reaction will be a desperate increase in terrorist activites, but after a while, they will realize that it is not working, and that they are only succeeding in provoking the deaths of thousands of their own countrymen and women.
In WWII, nobody believed that the Japanese would ever surrender. It took the horror of nuclear weapons actually being used to shock the Japanese into realizing that they could not win.
I feel the same thing will happen to nearly all terrorist organizations, if they see that the U.S. is not only capable of inflicting harm on their people, but has the political will to do so.
The only thing you should feel when shooting a terrorist: Recoil.