From the New Yorker via The Beast
The entire decade-long domestic death toll from terrorism (that is, where a political or ideological motive was apparent) was thirty. By comparison, the rate of annual deaths from mass shootings by non-ideological deranged killers—such as the gunman who attacked moviegoers in Aurora, Colorado, last month—runs more than thirty times higher (on average, about a hundred deaths each year). In all, there are about fifteen thousand murders in America each year.
Of the three hundred domestic-terrorism cases studied, about a quarter arose from anti-government extremists, white supremacists, or terrorists animated by bias against another religion. And all of the most frightening cases—involving chemical, biological, and radiological materials—arose from right-wing extremists or anarchists. None arose from Islamist militancy.