I'd give some weight to the shooter having had time in the area and on many routes.
Possibly was a bus driver, a limo driver, a taxi driver, a short-haul freight driver, or a member of a protection services team ... over the many back roads (as well as main roads) of Northern Virginia --- in the quadrangle from Winchester, VA to Lancaster, PA to Annapolis, MD to Petersburg, VA.
The shooter knows most of the back roads and very well.
The Northern Virgina area is full of protection services personnel and transport; they know how to get around; many of them are the support people for Middle-Eastern powers (gov't and private).
I would not underestimate the infiltration of this avenue, so to speak, by a terrorist, foreign or domestic.