Stop in for a cup of coffee

So, what I have read: He reported metal health issues, hearing voices and such. Was in a metal health hospital for a few weeks this summer. A firearms instructor. How is this all possible. Somewhere it said the police knew the guy, divorced 2x with restraining orders to boot. Nobody saw something bad coming?
As usual, nobody says anything to anybody, everybody that couldda done something didn't and here we are.