I don't usually defend bighead, but it was fair of him to assume it was probably Muslims. Extreme-right and left terrorists tend to hit targets they see as being part of whatever problem they're terroristing against (black churches, police stations, abortion clinics, etc.) instead of high-profile high-bodycount events like marathons. Radical Bostonite Muslims too lazy to blow something up in a different city, Radical Bostonite Orangemen too lazy to blow something up in a different city, or a mentally-ill unibomber with a serious grudge against the Boston Marathon are the only likely culprits.