The fact that it occured is precisely the reason we need to be damned sure it was really the act of a globally powerful organization and not just the coincidence of good planning and poor security.

But it's the fear instilled in us that it can happen again "at any time" that this documentary focuses on. Yes, they got us once, and in a way we couldn't possibly ignore. Did you know not a single person we've captured in the wake of 9/11 has been convincingly tied to Al-Qaeda, and that the name "Al Qaeda" was only coined after 9/11 because it was legally necessary to have a named target in order to prosecute the uncaptured bin Laden? The War on Terror presupposes a globally-connected terror network. There's simply no evidence to support the existence of such a network.
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"I wish I had documented more…" said nobody on their death bed, ever.