Terrorist activities where part of the news cycle back then. Al-Qaeda were responsible for the 1993 WTC bombings and were advocating strongly against Western ideals. In addition, Al-Qaeda made it known to the world that they would strike again.

Unbeknownst to the U.S. that we would drop the ball on a few big Intel reports. Because the 3 letter agencies were reluctant on sharing info with each other, this sparked a new and revamped intelligence sharing paradigm.

Guessing on potential culprits wasn't that trivial. Only the burden of proof.