Or, maybe the shooter was never in a white van to begin with, but once this idea became public, he waited until a white van was in the area before shooting (then he drives away in his red Mustang or whatever).
It is a distinct possibility that there is no "white van" associated with these murders. Since the description of a white van has been choking the media airwaves for days, and the perp(s) obviously are aware that a white van would draw immediate suspicion at or bear the crime scene, and that such a vehicle would never get through a roadblock, it suggests that only severely brain deficient criminals wouldn't consider switching vehicles or stealing a new vehicle for each new murder attempt.
I hope I'm wrong and they do catch him (them) in a white van. It would certainly make the job of the police a lot easier.
My only experience with rifles was when I worked for the Alaska Department of Fish and Game as a Game Biologist. I used to monitor the walrus hunts off the coast in the Bering Straits. I carried a Weatherby 300 Magnum with a scope. Even when it was zero'd in it was not easy to assure hitting a target at long distance. Of course, when it did find it's target, there wasn't much left as it exited. This guy must be very good and have some excellent equipment to "one-shot" these victims.