A new FBI report on active shooters says that mass shooters don’t explode out of nowhere—they plan and prepare. Most have no diagnosable mental illness. And their attacks are almost always preceded by behaviors that stir apprehension in people close to them, possible warning signs that crop up over a lengthy period of time.

https://www.securitymagazine.com/articles/89153-fbi-report-details-commonalities-among-active-shooters