Let's just say the perps are middle eastern and be done with it. At the very least it narrows the demographic.
What I don't get, if they are jihadists (which seems likely if there are more than one of them in the van), is why don't they move around the country more, to find places where there is less surveillance?
And on that note, why haven't more traffic cams been put up around DC? Even if the cams aren't everywhere and close to a shooting, a statistical analysis after several shootings would probably catch these guys. (Of course, they could be rotating phony plates.)