In any case, it's pretty hard to get into people's minds. There's no "background check" for that. Mohamed Atta and his toadies didn't have any questionable backgrounds except that some had overstayed their student visas. And box cutters were not banned items from carry-on luggage.
A person with a squeaky clean background could one day be transformed into a suicidal religious fanatic. It's happening every week in the Mideast; ...whether Picture I.D.'d and "background checked" law abiding citizen or not.
Trusted FBI counter espionage agent Hansson with top secret clearance and multiple "background checks" and "polygraph checks" one day had decided to spy for the Soviets. And he did for 18 long years!
Any attempt to bulletproof security is impractical reality. We can take reasonable precautions. Because since the dawn of humans, and religion; war and terrorism have been part of the equation. Life includes an element of risk.
