I do now that you have cited other instances etc. I agree with you that there may be an element of racism in some Americans attitudes, however, with our history of Irish terrorism in the UK, most of us have no problem imagining a white person as a terrorist.
In due respect that is incorrect. A terrorist has political ideals which he wishes to achieve through his actions. A lone shooter is often holding a specific grudge which motivates the action. They are not the same. Skin colour, incidentally, is absolutely nothing to do with it.