From the Document: "The FBI has designated 27 shootings in 2018 as active shooter incidents. As with past FBI active shooter-related publications, this report does not encompass all gun-related situations. Rather, it focuses on a specific type of shooting situation. The 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 inherently implies that both law enforcement personnel and citizens have the potential to affect the outcome of the event based upon their responses to the situation. [...] The findings in this report are based on publicly available resources, FBI reporting and, when available, official law enforcement investigative data. Though limited in scope, this report was undertaken to provide clarity and data of value to federal, state, tribal, and campus law enforcement as well as other first responders, corporations, educators, and the general public as they seek to neutralize threats posed by active shooters and save lives during such incidents."
Federal Bureau of Investigation: https://www.fbi.gov/