The nagging question is - Where are they hiding the rifle within the van? A false panel? Probably relatively barren inside the van to speed any search along. The cops open the back - see a neat van with little inside - and wave them through.
I still keep going back to idea that this guy goes to his shooting position (outside and far from hit zone) much earlier, and lies in wait for a victim to pass into his hit zone. Then he shoots, packs his stuff quickly, leaves the area in opposite direction from the commotion he has caused, gets into a vehicle (not a white van/truck - just a coincidence that they are seen in area - tons of them on the road, that you could look up at any given moment and see one in traffic) and departs.