I really wanted to make a broader point about the people closest to these shooters being the ones most likely to see patterns of alarming behavior. Being threatened by someone brandishing a firearm (or "letting you know they have one") is pretty serious.

What makes it really tough is these shooter seem like (to me, at least) they are on suicide missions, so patterns might not even exist. As far as I know, none have had any escape plans, hideouts, etc. prepared. Just grab the guns one day and head out.