From the Introduction: "The FBI has designated 28 shootings in 2019 as active shooter incidents. This report focuses on active shooter incidents--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 incident based upon their responses to the situation."
Federal Bureau of Investigation: https://www.fbi.gov/