There's very strong evidence to show this was "let to happen". I would challenge anyone to say otherwise and back it up. These offending individuals weren't even placed under the most basic surveillance after our security agencies received very precise tips about their activities related to this plot. Again, they did not track these very same individuals that would board those planes.
And once the attack DID begin, they were allowed to continue for as long as they needed. Think about that. They were allowed to continue for as long as they needed, unstopped for nearly an hour and a half. Think about those undeniable and fundamental facts in this case.
We must face this reality. The evidence says this was at the very least allowed to happen.
So when you slowly and painfully accept that it was allowed to happen, and you consider the required intent involved to "let it happen" in this murderous case, it's just a very small step to assume that it was helped to happen or even made to happen. It's called plausibility. Plain and simple.
THEN when you take it into the realm of hindsight, and the wrenching changes we've suffered as a result: the backwards excuse for "patriotism", the unspeakable loss of our rights, the open-ended wars, the compromising of our most basic values as Americans, the funneling of money to the tune of trillions of dollars, and of course the kneejerk manipulation that has made it all run so smoothly, it should become painfully clear to any rational person.