Let's consider this possibility while we're waiting for more information/evidence: Mass Shooters
It's an interesting article in the WSJ by Ari Schulman on the profile of mass shooters and how to stop them.
yeah.......but I am more inclined to think that this incident was more personal and may not have been a mass shooting..
In other words, he had a target and a motive.
Mass shooters kill everybody and anyone and then usually themselves.