I agree we do not want these type of people onboard in the 1st place, but on what premise are you going to deny access?
You would have to vet billions of people (impossible)and check on their past histories.
Then you have to exact proof of ID,again impossible.
I have been on 2 flights in the last 15 years with groups of Arabic / N African passengers behaving very suspiciously,
-like something out of a movie.
In this situation, we had no idea of their origins , intentions , and their purpose for being onboard.- You can't stage an interrogation mid-cabin on pushback, just because you have a hunch.
(These guys in both circumstances, moved frequently around the cabin , and were spread throughout the aircraft, passing hand-signals and mouthing expressions to one another.)
It is very difficult to find a solution, and I would think almost impossible to create a list of known terrorists/sympathisers, and have this information quickly accessible to all airlines at short notice.
Theres a good solution somewhere, I hope someone finds it and implements it before we have another such tragedy .