Mandatory reporting
My understanding in most mass killing events or attempts there has invariably been a trail that is later unearthed where every body joins the dots and says we all knew he (never a she) was going to do it.
This man is no different in my view, there is plenty in his history that should have rung alarm bells. With each day more of that history is coming to light this should have been disseminated
The so called sick notes in his apartment are just a small part of it, but addressed to to the wrong person.
Mandatory reporting is not uncommon, for example in Australia it is mandatory for selected classes of people involved with children to report signs of abuse. In previous posts the Swedish model has been well outlined in relation to aviation
I appreciate that this is not a popular suggestion but it has to be faced.