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.
For every person that has left a trail like this guy did, there are a million who have left a more troubling trail and never hurt a flea.
People always say they had a feeling or blah blah blah after the fact. He was weird. He was a nice guy but there was something about him.