It well might be a lone sniper with ME sympathies but I'm still more inclined to think it is a cell. If not you can be certain that terrorist cells are keeping close tabs on the situation and taking notes on how we react and where the flaws are in the system.
Does anyone else remember reading recently about what was thought to have been a terrorist training camp in Alabama ? It was in an old rock quarry or something like that ? There was an old school bus and a police car there, both were all shot up.