Even if the CIA had advance warning of an 'imminent attack', what do we expect it should have done with NO SPECIFICS given? It would have taken time to track down info on suspects, and it wouldn't have had jurisdiction in the US anyway, that would be the job of the FBI. The FBI was trying to find some suspected terrorists it KNEW to be here, but this is a big country with freedom to go anywhere, so with a head start, the terrorists were always ahead of the FBI.
This is 20/20 hindsight. Yes, we can say they should have known, or they should have told us, but the FBI, as well as the CIA, gets thousands of threats on something or somebody everyday, and that's a lot of threats to track down and investigate.