I agree with EROS also. And let's not forget, we don't know for certain what type of gun this nutcase is using.
As for a terrorism link, something did come to my mind while watching TV coverage last night of the road shutdowns.
What has happened immediately after the last two shootings? Police have swarmed to the area, and focused (rightly) on the scene and on stopping traffic.
Perhaps this is just my over-military mind going to work, but I can see a terrorist process that would get police in the habit of responding this way, and leaving vulnerable buildings or businesses open to a truck or car bomb attack.
Obviously to be successful with such a plan, the killer would have to move closer to DC and other areas with concentrations of federal buildings, but it is just a thought that is going through my head.