Well, what happened was one of the supposed hijackers got a bit too drunk in a nightclub in Fort Lauderdale and "blabbed" unknowingly to an undercover FBI agent working on another case. That perked the interest of US legal authorities and quietly, the FBI, working with the New York and Massachusetts National Guard, set up a "sting" to stop the hijackers the day of the attack: September 11, 2001.
What they found after the hijackers got arrested was unusual: pilots who trained to take off, but never land, Boeing 737, 757, 767 and MD-80 airliners and a LOT of box cutters that was supposed to be used as weapons. Their ringleader, Mohammed Atta, confessed he was part of al-Qaeda and got funding from groups associated with them. The fact most of this ring were Saudi nationals ended up causing a HUGE embarrassment for the Saudi government, and let's just say a number of Saudi government ministers got "sacked" or mysteriously retired.
But it still resulted in much tighter security restrictions, though. Security screening became MUCH more thorough, and for a while they even banned anything that had a battery in it on airliners until better screening equipment became available (e.g., you can't bring a cellphone, portable music player, let alone a laptop computer, on a plane).
With revelation that Osama bin Laden nearly pulled off a horrible terrorist attack, efforts increased to either capture or kill him. That was accomplished on September 25, 2001, when F-15E's operating from Diego Garcia used the GBU-28 "bunker buster" bombs and permanently sealed the caverns where bin Laden was hiding at the time in southern Afghanistan--those collapsed caverns are now his tomb....