PART 1: A Study of the Pre-attack Behaviors of Active Shooters in the U.S. between 2000 & 2013 ("Phase II Study")PART 2: A Study of Active Shooter Incidents in the U.S. Between 2000 & 2013 ("Phase I Study")PART 3: A Study of Active Shooter Incidents in the U.S. in 2014 & 2015PART 4: A Study of Active Shooter Incidents in the U.S. in 2016 & 2017The FBI defines an active shooter as one or more individuals actively engaged in killing or attempting to kill people in a populated area. Implicit in this definition is the shooter's use of one or more firearms. The active aspect of the definition implies that both law enforcement personnel and citizens have the potential to affect the outcome of the event. In 2017 there were 30 separate active shootings in the U.S., the largest number ever recorded by the FBI during a one-year period. With so many attacks occurring, it's easy to think nothing can stop a determined active shooter. "They just snapped" and "No one could have seen this coming" are common reactions that can fuel a collective sense of helplessness. But there is something that can be done. In the weeks and months before an attack, many active shooters engage in behaviors that may signal impending violence. Some of these behaviors are observable and may lead to a disruption prior to an attack.By articulating the concrete, observable pre-attack behaviors of many active shooters, the FBI hopes to make these warning signs more visible and easily identifiable. This information is intended to be used by law enforcement officials, mental health care practitioners, threat assessment professionals, parents, friends, teachers, employers and anyone who suspects that a person is moving towards violence.In 2014, the FBI published a report, referred to as the "Phase I" study, titled "A Study of Active Shooter Incidents in the United States Between 2000 and 2013" [Part 2 of the present volume] identifying 160 active shooter incidents in the U.S. The same methodology was used for the following years, identifying 20 incidents in 2014, 20 in 2015, 20 in 2016, and 30 in 2017 [Parts 3 and 4 of the present volume]. In the reports covering 2000 through 2017, we focused on the circumstances of the active shooting events (e.g., location, duration, and resolution) but did not attempt to identify the motive or pre-attack behaviors demonstrated by the offender. In 2018, the FBI published its "Phase II" report [Part 1 of the present volume], moving from an examination of the parameters of the shooting events to assessing the pre-attack behaviors of the shooters themselves. This second phase, then, turns from the vitally important inquiry of "what happened during and after the shooting" to the pressing questions of "how do the active shooters behave before the attack?" and, if it can be determined, "why did they attack?" The FBI's objective here was to examine specific behaviors that may precede an attack and which might be useful in identifying, assessing, and managing those who may be on a pathway to deadly violence.