Originally Posted by
alanR
So a terrorist from a visa waiver country or someone "adopting" the identity of such is used.
Then there's the ones who have immigrant visas, green cards or who are citizens of the US...
Impostors may be a problem. But with more biometrics that is getting harder. Alien Resident Cards and US passports present a challenge to POE inspectors. Every visa issuance is vetted against the lookout system. Visa waiver country travelers to the US are subjected to an ESTA clearance before they fly.