Not entirely true that all extremist Muslims are from the Middle East. There have been a few failed bombings in London recently, a flaming Jeep drove into Terminal 1 at Glasgow airport a few days ago - carried out by English Muslims. The 7/7 bombings in London a few years back were also English Muslims.

I guess that a number of extremist Muslims in the US were born and raised in America, and had the idea planted in their heads to attack by visiting Muslims, or those that went over to live in the US.