As to technical ability presuming the distance is about 100 yards or maybe less I would expect several million people would have the technical ability to hit a target the size of a human torso at 100 yards. Just about anyone could with a little practice be able to accomplish this. As the distance goes up the difficulty of the shot increases exponentially. With a year of training I would expect a large number of people could with optical sights make a 500 yard shot on a human torso sized target with a high degree of reliability. If the training was intensive a month would be more than sufficient.
Stay well - Stay saf3e - Stay armed - Yorktown
They will ulitmtely find what Yamamoto knew about trying to attempt any ground war here ... even a guerella one. Too many people armed and in the face of "open season", initial liberal/socialist clamoring for gun control will turn into a reverse open season on those doing the shooting ... again, if it is Islamic terrorism.
The shooter(s) favors the Maryland area, undoubtedly very familiar with it. It would be interesting to know if any ME types have been taking lessons at shooting ranges. Believe it or not, they have 50 shooting ranges in Maryland: bang
Of course, that would be racial profiling...