It's actually harder to spot paranoid schizophrenic or psychotic tendencies among foreigners immersed in rarer foreign languages and exotic customs. The craziness is often just shrugged off as part of a "different culture." And the liberal trend is to be "tolerant" of this.
If any of us had seen, say, Mohammed Atta walking down the street before 9/11 what would really reveal that this was a raving homicidal lunatic and not just another "weird" foreigner acting a little "oddly"?
I'm sure there's truth to that, but:
I don't know if we would have noticed much in Atta's behavior anyway. I don't think those guys were crazy in the "I have to take this bucket of cheese to Washington DC before the aliens pick beat me to it"-type way. They blended in quite well. Don't forget: the enemy is far from stupid. They knew who to send on such missions.
And who saw those in Gacy or Dahmer? If anything, the Hmong community would be far tighter and more apt to notice such things within among each other. I sure as hell don't watch other white people, or anyone else outside of my immediate circle. Do you?