There is absolutely no way to do a background check on a foreign man from a backward country where 100% of population hates you for one reason or another. What will the checking agencies do - ask for a recommendation from his Mullah? How would anyone even track a person in a country without written records? The only thing you can report about such a man is that he hasn't killed anyone yet, to the best of your knowledge.
Exactly, and to boot, these guys are following a religion that encourages one to befriend and lie to the infidel to get close enough to further the cause...
"They’ve been vetted, just like the other guys that turned around and killed Americans?"
Vetted, just like the current occupant of the oval office? ...the Fort Hood shooter? Really?!