For some reason there is some debate over whether the mass murders at the Charleston church should be called terror or just crazy.
What's the difference? For me the difference is motivation and organization. The crazy guy may imagine that he is responding to instructions from God, the Devil, or Slender Man, but the terrorist is responding to an actual political movement and organization.
Most of the domestic terrorists/mass murderers we have seen in the US seem to have at least a toe in both camps. Most are troubled loners, not recruits or apparatchiks, but they draw at least some of their inspiration from organized hate groups. Dylann Storm Roof, the Charleston shooter, appears to follow this pattern. Most of the school shooters, and the Denver theater shooter, by contrast, appear to fit more into the purely crazy mode. A few, like the shoe bomber and the Times Square would be bomber, look more like politically motivated terror recruits.
Intelligence and surveillance sometimes work against the last category, but they are all but useless against the first two types. What would work is keeping guns out of the hands of crazy people - something that the NRA fights fanatically against.