Vetting can be impossible where the US has no access to local records or where local records have been destroyed. 
How, for example, is the US able to determine if a cousin or an uncle is not an enemy combatant pretending to be just an innocent Muslim civilian?
Is the US developing a profile based series of interviews similar or identical to what the IDF (Israeli Defense Force) employs on passengers boarding an El Al flight to get an idea at the character of the person interviewed in the absence of capability to do a background check?