The town is Massaponax (just get hooked on phonics and the pronunciation is easy). Four Mile Fork, which is what the specific area is called, is along U.S. 1. It's just a few miles south of Fredericksburg, about a mile from an exit of I-95. (Gimme a minute and I'll post a map)
I can tell you this, though. If they tried to escape heading north on U.S. 1, they picked the worst possible escape route. Traffic really sucks there.
UPDATE 10:11 a.m. EDT
Someone apparently eyeballed the shooter, not just his vehicle, but the shooter. We're getting conflicting reports about the pursuit. They have him going in different directions. The most solid indication, however, has him heading north towards Quantico, Va. (which jibes with the original chase report. U.S. 1 goes right to Quantico from Fredericksburg).
That way the shooters' vehicle could take off slowly with the evidence and not draw attention.
It's a possibility that's why more than one vehicle is being sought.
If they don't get this scum, you folks in Manassas and Leesburg watch out later today. In fact, if you're in Fairfax, Montgomery or Loudoun, you'd best go fill up now... shooter can't physically make it there till about 11.