On other note, I have to concede that my earlier speculation of UN/maryland link was pretty obviously wrong. That said, the Maryland thing is highly unusual in a way that means we have to call it at least a 50/50 proposition that Islamofascist terrorists are involved.
The sonic crack which is left does not point to the shooter.
If the shooter is competent, he'd move around a corner (maybe 40 yeards) from the van, shoot on foot, then run short distance back to van. This would reduce odds of van being spotted, also reduce odds of his getting caught between hits compared to purely pedestrian affair.