The one witness account describing an accomplice seems suspect from the common-sense perspective that sociopathic behavior on this scale is rare. Terrorists aside, to have two people comfortable with random killing is unlikely. The Columbine school shootings were committed by two like-minded perpetrators, but the victims were not chosen completely randomly. On the other hand, a man with a rifle who sets up and waits for a shot in densely populated suburban Maryland will likely attract attention. An accomplice who could watch out for interlopers and assist in flight would make these attacks much easier to commit.
Ultimately, the witness description about a small box truck likely provides the best chance of catching the sniper. In addition to providing him with the ability to shoot from a concealed place, a truck would allow the sniper to carry off any evidence that he might otherwise leave at the scene, such as more spent shells and foreign fibers. Vehicles are easy to trace, and when police do find the truck, it will contain the evidence that the sniper didn't want to leave behind. But given our sniper's skill with a rifle, let's hope that when police do find him, he doesn't see them coming.