the start if an unverified audio message. this is the latest example of extremist terror, and we have to fight it with everything we have. reporter: growing u.s. worries about the spread of isis and where it could lead. a terrorism attack like this one on a soft target in tunisia promises there will be more terrorism in europe and eventually will wash up on the shores of the united states. reporter: for now, the u.s. doesn t doubt the isis claim of responsibility. but officials caution al qaeda affiliates in north africa also are very active in tunisia. these attackers perhaps inspired by isis rather than directly ordered to attack. violent as it was, u.s. officials say the tactics appeared haphazard, not a typical terrorist plot.