U.S. government foreknowledge of the 9/11 attacks?

As I’ve said in the past two posts, what I personally find suspicious about 9/11 is not the story of the hijackers, but rather the claim that the attack could not have been prevented.

It seems to me that the FBI and the CIA had no good excuse not to have known about the plot in advance, in detail, given that the plot had been hatched way back in 1995 or so, and given that the FBI and the CIA had indeed been spying on al-Qaeda for lo these many years, except that the FBI sometimes refused, for no good reason, to follow up on obvious leads.