One of the witnesses apparently described the gunman as a man with a "9/11" tattoo. It would appear part of the problem is that people in America can't even get their bigoted ignorance right "Durrrr, they're brown and wear Turbans- they must be Muslim and therefore evil!"
Terroristic acts on Mosques are sadly not that rare in America, they're just not usually reported as terrorism by the media, on the rare occasions they're reported at all. (Rare beyond the very local news at least.) It's the same kind of subtle anti-Muslim bias which leads to Sunni and Shia Islam always being called "sects" instead of "denominations". Two words which mean the same or nearly the same thing, but if the News said "there's inter-denominational violence" instead of "sectarian violence" it would make the victims of said violence too human and familiar to American Christians. *sigh* Human beings, always striving to define those not in their group as inferior and alien, it's a damn shame.