Policies and inaction invited this attack.
Two Americans, in particular, failed to see the implications of their enforcement policy and inaction. Books have already disclosed who they are, and In the interest of decorum, only one title is listed here:
Dereliction of Duty
The second scoundrel not only caused critical details to be withheld from the captured laptop computer belonging to the "twentieth hijacker", but created the US' Housing Crisis of 2007!
It even gets worse than that.
Anyway, we're unlikely to be attacked again, as the numbers of people needed for such terrorist actions haven't presented themselves.
Oh, wait...