was motivated, if the FBI is to be believed, by Islamic extremism. Can you think of anything the U.S. is doing that might have upset these people over the last decade or so?
Yes, they were living here, but that is no guarantee against terrorism, see Timothy McVeigh and Eric Rudolph.
Now those two examples would seem to point to people our foreign/domestic policy isn't hurting, but actually, that is not true. McVeigh was a veteran of our first war in Iraq, and Rudolph was kicked out of the Army for weed. Thus our failed policy of foreign intervention and our failed policy of drug interdiction, along with our failed policy on veterans mental health care and employment, and our failed policy on investigating domestic terrorists (anti-abortion religious fanatics are given a pass and rarely called terrorists) was influential in what these men did.
And, of course, you are right. Some people are just insane. But then that would fall into the category of our failed policy for dealing with the mentally ill.
So, if you want to stop 99.99% of terrorism:
1) Stop killing other people
2) Stop meddling in other people's affairs
3) Take care of sick people
4) Take care of the people you train to kill people (soldiers and police)