Rustigan sees the point: "This is a clean, meticulous, methodical man. A card like that, 'I am God,' is a bit flamboyant for this clinically detached marksman."
However, Rustigan sees the sniper falling into a slightly different category from the likewise detached Unabomber. While the Unabomber seemed to pick his victims and write as a way to voice his beliefs, the sniper's victims seem to have been chosen at random. For him or her — as with rage-filled, single-incident mass killers — "simply being human may be enough" to become a target, Rustigan says.
"This guy's probably a run-of-the-mill disgruntled loner with loathing toward people — a guy that hates society, hates the system, a guy that's fallen down and he's mad as hell," Rustigan says. "Rather than shooting up a cafeteria … he's going to be clever and it's going to be one by one. He's going to generate a hell of a lot more publicity by doing it individually.
"When a guy bursts into a restaurant and kills a bunch of people, it's one burst of manly glory. This guy is achieving manly glory, in his mind. But at the same time, he's carved out a career. He's made a name for himself. You know, 'The guy's a hell of a shot. He can kill anybody at any time.' "
ABCNEWS' John Miller and Andrew Chang contributed to this report.