The Quantico base, which is about 30 miles south of Washington, DC, is famously known as the primary training ground for both the Marine Corps and the FBI. It's referred to as the "Crossroads of the Marine Corps," since most officers and soliders who receive advanced training pass through there at some point during their careers.
The two victims were also Marines, but their connection to the shooter has not yet been revealed. The only other information given about the shooter is that he was a staff member at the officer candidate training school on the base.
Update: A Marine spokesperson has since denied the earlier reports of a standoff, saying that "it is not the situation that there was a standoff or barricade,” but it took some time to find the second victim and the shooter.