First thing I said when I heard a description of the van was just what you said: skilled marksman using a scoped rifle on a bench.
The majority of people who have spoken to the press have said that they didn't hear the shot but reacted to the victims falling down. Some mentioned thinking the victims were having heart attacks.
Three counties have shut down all outside activities for the afternoon and evening. Schools were locked down all day. And the evening commute will be a nightmare with LEO pulling over all suspicious vehicles in the DC/Balto. region. Also there are reports of roadblocks and checkpoints.
The local government mouthpieces can call it what they want but the effect of the shootings is terror.
In addition a paramedic buddy says that at a certain point the first responders will not respond without an escort. Yet another intended consequence.
Arm everybody.