And you know this how?
I know this because dialects are not easy to discern unless a person is a native speaker of that language. I know this because, as the subject of the thread points out, they (Al Qaeda) have ALREADY successfully made it through in large numbers... so apparently it's not obvious to someone. And I know this because Mexico itself has different dialects, and unless a person is a true expert, how would our Border Patrol know the difference between a Mexican dialect and a non-Mexican dialect? And while I recognize that you are probably a language expert, based on your previous post, how many of our Border Patrol do you personally know who are as proficient as you are at discerning different Spanish dialects?