There was the millennium plot of 2000 that was thwarted and even in the immediate aftermath of the '93 trade center bombing officials were warning that they would strike again (Khalid Sheik Muhammed was the brains behind both attacks) Plane hijacking had been the go to terrorism thing since the 70s for any and all groups despite ideology so it wasnt really as precise as it seems in hindsight. The reports from right before the attack all show that they knew something was up but they couldnt figure out when or where it would happen.

A thing that annoys me about truthers is that none of them seem to ever study the history of the attacks, they all just look at how dust hit the ground