Yes, it appears that a good percentage of terrorists are drawn from the home country, and not necessarily from overseas. Sleeper cells, is the word you'd be looking for.
People have become stereotypically induced to believe that terrorists are - suicide bombers, from the middle east, bent on using explosives strapped to themselves, or using vehicles as a kamakaze means.
Terrorists include those not brainwashed into Wahhabism (which is the extreme interpretation of Islam, of which about 10% of the muslim population follows) - a clear and even local example (for you americans) is the Oklahoma bombing of 1995.
Those gentlemen were indeed terrorists, but they have no links to Islam, or Wahhabism, or any of that stuff.
"It is the deadliest domestic terrorist attack in the history of the United States and was the deadliest act of terrorism within U.S. borders until September 11, 2001. "
He sure looks Middle Eastern to me.
