Yes, the F.B.I. could have stopped 9/11. It had a warrant to establish surveillance of everyone connected to Al Qaeda in America. It could follow them, tap their phones, clone their computers, read their e-mails, and subpoena their medical, bank, and credit-card records.
As the March on Washington is remembered five decades later, it should be noted that King’s successes occurred in the face of direct and underhanded opposition from forces within the US government, most of all, Hoover, who did not hesitate to abuse his power and use sleazy and legally questionable means to mount his vendetta against King.