Eyes on the ground in Boston could see that there was a threat prior to 9/11:
At least three eyewitnesses spotted al Qaeda hijackers casing the security checkpoints at Boston’s Logan Airport months before the 9/11 attacks. They saw something and said something — but were ignored, newly unveiled court papers reveal.
One of the witnesses, an American Airlines official, actually confronted hijacking ringleader Mohamed Atta after watching him videotape and test a security checkpoint in May 2001 — four months before he boarded the American Airlines flight that crashed into the World Trade Center.
The witness alerted security, but authorities never questioned the belligerent Egyptian national or flagged him as a threat.
Adding insult to injury, they weren't even very good at casing the place. They were obvious, loud, and belligerent.
Yet locals in positions of responsibility still failed to take action.
Really, our response to 9/11 has been to scrutinize everyone using a massive bureaucracy of agents when just being observant and reacting to suspicious behavior would probably do the job.
Tip to Instapundit.