This document defines an active shooter and outlines characteristics and responses. It begins by defining an active shooter as someone actively killing people in a populated area without a clear victim selection pattern. It describes historic shooters and notes that shooters often have feelings of alienation and persecution. The document outlines characteristics like anti-social behavior, aggression, and relationship issues. It discusses the 5 phases of active shooters and recommends notifying authorities about concerning behaviors. For responses, it recommends evacuating if possible, locking down if not, taking action against the shooter as a last resort, and cooperating with first responders.