They’ve been vetted, just like the other guys that turned around and killed Americans?
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.
Who,s got their eye out for Mullah Omar?