Turns out (after several right-wingers on Twitter tried to blame Middle Easterners) it's a veteran who had been having PTSD problems and several run-ins with the law.
My question is... why'd the guy still have his firearms?
I'm for gun ownership, but if somebody's having emotional problems bad enough to need treatment, and if he's had altercations with the law and is "known to" authorities... then for his own good and the good of all around him, take his fucking guns away, at least until he gets that shit squared up. If it's just a bad patch and he gets past it, fine, but at least for the time being, take the guns, the way you pull someone's driver's license if they get D.U.I.'s. We're fine on regulating cars because they can be dangerous, but guns (A) are designed pretty much only for killing things (even though you can target shoot, but that's pretty much just practice to be able to kill better), and (B) aren't as needed in everyday life as cars. I like guns, but many people get through their entire lives fine without ever owning one. It's more difficult to do that with a car.
Because nobody thought "Maybe this guy shouldn't have access to firearms for a while," we have a bunch of dead kids, a dead law enforcement officer, and a dead veteran who had earned a noble legacy for his service but ended up disgracing himself beyond all measure... all because nobody said "It'd be reasonable to not let him keep his guns while he's in this emotional state." They knew there was a problem. They were, apparently, just timid about doing anything about it. They gambled and everybody lost.
Frustrating, and tragic. And likely could have been avoided.