Some of those people who are "here" haven't been here long and come from cultural backgrounds that are steeped in hatred for the west and America in particular.
There is such a small percentage of "white" native born Americans who have done this and most are complete wack jobs and not driven by their God to perform such heinous acts of depravity. Percentage-wise there's been a greater number of attacks from fanatic Islamic-inspired theology believers compared to Islamic theology believers, than the percentage of white privileged CIS cracker knuckle-draggers compared to the overall population of white privileged CIS cracker knuckle-draggers.
We're told all the time, well just because there's a small number of Group A is acting up, it doesn't mean ALL members of Group A are bad. And, sure just because these guys are part of Group B and did something awful, it doesn't mean because they're a member of Group B that they did this. It was something else. Perhaps a jilted lover, homosexual confusion, they're off their meds, they've doubled up on their meds, they ate at Mighty Taco and got a-getta, or they had a case of the runs and s hit their pants and got in a mood that matched the foul smell in their britches.
If you're looking to stop trouble before it happens, personally I think it's a better spend of scarce resources to look at the fomenting bags of puss in society and focus on those groups. Instead, we get TSA pat-downs of 80 year-old grandmothers from Minnesota in airports.