It's pretty easy to sit back and Monday Morning Quarterback the intelligence community after the fact. People can gloss over all the information in hindsight and find that one indicator that "it" was going to happen and point to it and say "see, they KNEW about it!" And I say again, if someone had told me on September 10th 2001 that terrorists were going to fly airliners into the WTC and the Pentagon, I would have thought they lost their mind and needed professional help. And I'm probably not alone in that assessment.
Just like people can look back at the indicators of WWII and say "they KNEW it was going to happen!" It's no secret Roosevelt wanted us in the war, but I don't think he wanted 3,000 Americans to die in order to do it. Nobody thought that an attack on Pearl Harbor could have come like it did and had you told the majority of the public the Japanese had come up with a special kind of torpedo that wouldn't bottom out in the harbor, they would have laughed at you.
Again, there were probably indicators of the attacks in advance. Maybe even details that were so unbelievable that they were dismissed. Our intelligence community took a black eye over the ordeal and I don't see the need to rake them over the coals because something was so far out there it was hard to believe.