I suspect this may be research by a terrorist cell or sympathizer(s) because (a) the care taken in getting in and out of position, (b) the selection of targets challenge each thing the newscasters observe hasn't been done, i.e. to maximize the terror impact, (c) it is happening in the most secured area of the country, i.e. Washington area - as if a test of the worst case scenario, (d) the discipline to limit each attack to a single shot, (e) the timing of other world events.
To me, the attention to detail smacks of the anthrax attack - a controlled statistical sampling - the kind of approach I would expect more of Iraq than al Queda, though it could be one and the same. I would not be surprised to see the same end result - no visible success, but a lot happening behind the scenes through the intelligence community.