Kind of what I've been thinking. No group is taking credit for this which they would surely have done if this was a politically motivated terrorist attack.

Sounds to me like a particularly malicious suicide. Guy may have been passed up for a promotion or assignment he wanted, his girlfriend left him - anything to make him think "I'll show them!". Pretty psycho way of thinking to take so many with you but suicides aren't usually thinking straight to start with.

I'm sure the investigators are going through his stuff and his entire life right now looking for any clues as to motive.

Gunnery, gunnery, gunnery.
Hit the target, all else is twaddle!