2017 National Crime Victims' Rights Week Resource Guide: Crime and Victimization Fact Sheets: Mass Casualty Shootings [open pdf - 252KB]
"Mass casualty shootings are rare compared to other forms of homicide; however, due to extensive media and policy attention, as well as the number of people affected by each event, they are an important subset of crime. Measuring mass casualty shootings is complicated by the absence of a commonly recognized definition. Federal agencies define a mass casualty shooting as the murder of three or more individuals. Due to the rarity of these events, federal agencies do not collect data on mass shootings, but they do collect data on 'active shooters.' Active shooter incidents are defined as incidents where 'one or more individuals actively engage in killing or attempting to kill people in a populated area.' This definition does not include gang- or drug-related incidents, the accidental discharge of a firearm, or family- and intimate partner-related shootings. A majority of mass casualty research only accounts for people physically injured or killed, and discounts individuals who witnessed the shooting or were threatened by the shooter, even though they often experience psychological trauma as well."
United States Department of Justice Office for Victims of Crimes: https://ovc.gov/