You're not paying very good attention.
The way you PREVENT things like this is just that: Prevention. You find out about these things and you defuse the situation. You knock on doors in the early hours of the morning and herd half-awake, half-clothed would-be suicide Jihad-ers into waiting police vans.
This is how we have always prevented the majority of these tragedies. Even in situations like the Murrah building and the WTC bombing in '93, the intell community knew something was up but couldn't put all the pieces together in time to prevent what happened.
This time NO ONE KNEW!!! NO ONE HAD A CLUE!!!!
That's why we can say with confidence that our intelligence capability has suffered so drastically under Clinton.