You don't necessarily need the complicity of Bin Laden, or that many people.
You just need your intelligence agencies to keep track of what he is doing and allow him to continue because the attack suits your purpose (e.g. finding an excuse to attack Iraq).
That's a much more plausible version. It's still more complicated, though, and requires the complicity of many more people, and is therefore less likely to be true. There's also the rule of thumb that we shouldn't presume malice when incompetence fits.
You just need your intelligence agencies to keep track of what he is doing and allow him to continue because the attack suits your purpose (e.g. finding an excuse to attack Iraq).