Clancy predicted it one of his books.
Phillipino intel, Sudanese intel, and lots of uncollated evidence pointed to this.
The evidence found (that has been publicized) from Al Qaeda in Afghanistan points to a few things:
Dirty nukes, bioweapons, weaponized industrial gasses, black market nukes, indigenous nukes and homemade nerve toxins.
We now know they can operate cells in the US relatively quietly for years.
We now know they use internet, converted cargo containers, phony ID.
We now know they infiltrate and hide behind Western religious freedoms and use mosques and religious teachers.
We know that a few western converts are willing to completely betray their countries and act as soldiers and terrorists.
The attack was not out of left field.
It was out of our own multi-year complacency and arrogance; we were asleep, just as in December of 1941.
Remember that Pearl Harbor was predicted by a Naval Officer years prior to the attack.
Ed