Why did the NSA not know about 9/11 before the fact? I do not trust them. Good intentions are not enough, assuming they have them, which is far from obvious.
There were warnings before the fact, but they were ignored by the political class. Intel can only warn and inform; it cannot act alone. Political people have to initiate, act, or direct on that Intel. If they do not, then there is 911. And so DHS was born - FWIW ...