An interesting Pew poll and relevant to the discussion. “More than eight-in-ten American Muslims say suicide bombings and other forms of violence against civilian targets are never justified (81%) or rarely justified (5%) to defend Islam from its enemies”.
I would have expected the number responding “never” justified to be much higher. Extrapolate the data across the entire US Muslim population and I can see why there is concern/fear — “rarely, sometimes, and often” respondents totaled 13% and another 6% refusing to respond. 13% of Muslim Americans (or roughly 234K adults) believe violence against civilian targets is at least “rarely” justified and 8% (144K adults) believe the violence against civilians is at least “sometimes” justified.