Turns out that was wrong. The military now tells Reuters (linked above) that they have arrested Sgt. John Russell for killing five US soldiers and wounding others.
Per the Times:
The dead included two officers on the staff at the clinic — one each from the Army and the Navy — as well as three enlisted soldiers who were at the clinic. ...
Sergeant Russell’s superior officers had taken his weapon from him and referred him to counseling sometime over the past week, presumably because of concern over his mental state, [Maj. Gen. David Perkins] said. Somehow, he got a new weapon, which he used to go to the clinic and open fire, he said.
The episode has sparked military investigations into both the circumstances of this attack, as well as into how mental health services are being given for soldiers, the general said. Sergeant Russell was “almost certainly” on his third tour in Iraq, General Perkins said.
This is a very sad story, and we don't want to assign blame to anybody -- except, of course, the killer. And Donald Rumsfeld, who sold America (and America's gullible then-president) on the myth of a cheap, fast war, which has since required soldiers to serve extended and repeated tours.
But we do have to say, gently and with all due respect, to the superior officers who quite rightly took Russell's weapon away from him: You do realize you're in the Army, right? In a heavily fortified camp, inside a war-torn country? It's not like a guy can't find a gun.