The first line of defense against airborne terrorists should be the visa process! If no visa, you won’t fly. And to get a visa, you have to prove that your background is clean, and that you have no adverse associations. You must account for what you intend to do here in the US and whom to visit, etc., especially for applicants from “trouble” nations. If it doesn’t check out, you won’t get a visa, and you won’t fly!
If anything turns out fishy, the claimed contacts here in the US should be checked out. Thoroughly! Put the burden on the applicants to prove they are OK, and hold each country’s US Embassy responsible for verification and background checks before a visa is issued. Whatever time it takes, a month, 3 months, forever, – so be it.
To visit the US is not a right for anyone, it is a privilege, and you have to prove you are a worthy visitor! The burden of proof should be on the would-be travelers, not on us!