Intelligence is a partial answer, but it hardly helps if the terrorist recruits have no background. Not all Muslims are terrorists, but almost all terrorists are Muslims. If you read some of my other posts, you'll realize that I put much of the blame for the current problems on the West's policies toward Muslim countries (for example see my posts above regarding Professor Robert Pape's book and Ron Paul's TV interview). I'm just trying to be practical about stopping carnage on commercial aircraft