They say G.W. Bush and Dick Cheney did it. A big 'DING! COO-COO' to that! The islamic extremists have been at it since Robert Kennedy was killed by Sirhan Sirhan.
Re: "There must be one soul who was pivotal in not doing anything to prevent it. Somebody is to blame."
That's the human desire for simple narratives with heros and villains crying out, not a rational analysis of the real world.
The real world is complicated, responsibility is diffuse, and sometimes events happen simply because factors happen to coincide in a particular way. Yes, some people were negligent. Yes, some people failed to do their jobs properly because of social/political factors.
But mostly the 9/11 hijackers figured out a loophole in some bureaucratic doctrine (give the hijackers what they want. They'll eventually need to land to negotiate) which enabled them to do significant damage.
Thriftymaid 