Could 9/11 have been stopped before it happened?

Before 9/11, FBI Special Agent John O’Neill kept setting off alarms about the emerging terrorist organization al-Qaeda and their mysterious leader Osama Bin Laden. As a result of his warnings the FBI knew terrorists were planning a big attack and the CIA even had eyes on two of the eventual hijackers prior to the attacks.

So what went wrong? Why didn’t they capture the hijackers? What more could law enforcement have done to stop the attacks? And could they have been prevented?