Part of the fear was that U.S. intelligence had failed again to predict the attack.  The public did not know what would come next, nor did it believe that U.S. intelligence had any idea. A federal commission on 9/11 was created to study the defense failure. It charged that the president had ignored warnings. The focus in those days was on intelligence failure. The CIA admitted it lacked the human sources inside al Qaeda. By default the only way to track al Qaeda was via their communications. It was to be the NSA's job.

As we have written, al Qaeda was a global, sparse and dispersed network. It appeared to be tied together by burying itself in a vast new communications network: the Internet. At one point, al Qaeda had communicated by embedding messages in pictures transmitted via the Internet. They appeared to be using free and anonymous Hotmail accounts. To find Japanese communications, you looked in the electronic ether. To find al Qaeda's message, you looked on the Internet.

But with a global, sparse and dispersed network you are looking for at most a few hundred men in the midst of billions of people, and a few dozen messages among hundreds of billions. And given the architecture of the Internet, the messages did not have to originate where the sender was located or be read where the reader was located. It was like looking for a needle in a haystack. The needle can be found only if you are willing to sift the entire haystack. That led to PRISM and other NSA programs.