frank cillufo is a former white house counter terrorism official. >> the concern is could there be copycats in the u.s. >> you always need to be worried about copycats because there is an old saying, terrorists are not innovators, they're imitators. >> reporter: according to this report by the house homeland security committee, more than one-third of isis-linked terror plots have been aimed at the united states or its interests u.s. officials also estimate 30,000 foreign fighters have traveled to syria and iraq to join isis, including around 150 from the u.s. one intelligence consulting group believes about 40 have returned. those are the ones u.s. law enforcement officials are aware of. >> where are you from? >> the united states. >> reporter: but just last week american mohammed jamal khweis surrendered to kurdish forces in northern iraq. he traveled from the washington, d.c., region the syria to join isis, and he was not known to the f.b.i. >> what you're seeing right now is we're shaking every known bush. you're going to have a number that won't necessarily be on anyone's radar screen,