Assessing our vulnerability to terrorist attack remains difficult in part because the events surrounding the attacks of September 11, 2001, have not been thoroughly explained. In its investigation into them, the 9/11 Commission slid past many important questions, leaving them unanswered, and did not adequately challenge the Bush administration when it refused to cooperate or obfuscated its own actions. This has naturally spurred the various conspiracy theories that have set out to unravel what happened. Some address legitimate issues, such as the as-yet-unexplained fact that American intelligence had been tracking lead hijacker Mohammad Atta since the late 1990s but did nothing to stop him. But most theories run the gamut from the preposterous (the planes that crashed into the World Trade Center and Pentagon were drones) to the improbable (the WTC buildings collapsed due to a demolition charge).
The supposed tracking of Mohamed Atta is the Able Danger nonsense. Some good debunking in this article, although he's a little too quick to accept the supposed "warnings" of 9-11, and how they should have been passed on to the American people.