From the Introduction: "The FBI has designated 40 shootings in 2020 as active shooter incidents. 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 a firearm. The 'active' aspect of the definition inherently implies the ongoing nature of an incident, and thus the potential for the response to affect the outcome. The FBI works proactively to identify incidents that meet the scope of this study, using internal FBI holdings and repositories, official law enforcement reports (when obtainable), as well as open-source data. However, there is no mandated database collection or central intake point for reporting active shooter incidents, as exists for other crimes."
Federal Bureau of Investigation: https://www.fbi.gov/