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What is a Mass Shooting?

Updated: Feb 28, 2022


The term "mass shooting" has many different interpretations, but no commonly agreed upon definition. In this article, we explain the leading mass shooting definitions, and put forward the terms "active shooter" and "active shooter mass killing" as useful ways to understand and measure these unique crimes. Federal Bureau of Investigation (FBI) data is used to chart the net growth of active shooter and active shooter mass killing events from 2000-2020. We also provide a list of steps you can take to prepare against this threat.

What is a Mass Shooting?

The term “mass shooting” does not have a commonly agreed upon definition. Most mass shooting definitions are variations on the FBI’s 2005 description of “Mass Murder”, the killing of four or more people, excluding the killer, in a short time frame, generally in the same location, and with no cooling off period. Unfortunately, the differences between some definitions are significant, ranging from defining a mass shooting as the killing of four persons including the shooter, to the non-fatal injury of three people excluding the shooter. To further complicate the matter, some definitions include incidents stemming from factors such as drug and gang related violence, while others do not. Since there is no commonly accepted definition, news articles, organizations, and studies provide significantly different and often contradictory pictures of the subject. [i]

Many mass shooting definitions only consider fatalities. This means all non-fatal casualties are excluded. Even if many victims sustained life altering injuries, if a minimum threshold of injured ultimately do not succumb to their wounds, the event is not considered a mass shooting. The "9ine Ultra Lounge" shooting in January, 2020, for example, resulted in one person killed and 16 wounded. It is considered an active shooter event, but not a mass shooting by most definitions.

Due to the confusion surrounding what constitutes a "mass shooting", the author proposes the terms "active shooter mass killing" and "active shooter" as more useful definitions. Studying these two benchmarks together offer clear benefits and insights.

What is an Active Shooter?

An “active shooter” is defined by the US government as “an individual actively engaged in killing or attempting to kill people in a confined and populated area” with the exclusion of events such as drug and gang related violence. For a more detailed explanation of "active shooter", and the full list of exclusions, please see, "What is an Active Shooter?".

What is an Active Shooter Mass Killing?

In 2013, the FBI began tracking "mass killings", defined by the Investigative Assistance for Violent Crimes Act of 2012 as the killing of three or more people in public spaces. While the Act does not specify, the FBI interprets mass killings as excluding the perpetrator from the three persons killed. The FBI has since published numbers for active shooter events meeting the federal Act's "mass killings" definition for the years 2000 - 2020. These events comprise a minimum of one less killing than the 2005 FBI definition of "mass murder" and also exclude the same special cases excluded by the "active shooter" definition. The events, termed here "active shooter mass killings", are put forward as a useful, if imperfect, proxy for the hotly debated term of mass shootings.

How frequent are Active Shooter Mass Killings and Active Shooter events?

Mass shootings. US Active Shooter events and active shooter mass killings: 2000-2020. Mass shooting definition. Mass shooting statistics.

[ii], [iii], [iv], [v]

What can I do to prepare against this threat?

While there is no cure-all, there are still a variety of steps you and your organization can take to reduce the chances and severity of a potential active shooter or active shooter mass killing attack. Specific best-practices will vary by context, but some options to consider are as follows:

  • Implement an emergency action plan for yourself and your organization.

  • Establish a prevention group in your organization.

  • If you work in education, consider creating an anonymous tip line.

  • Consider implementing a mobile emergency communication system for your organization. In a crisis, a well-designed emergency application can be critical in helping you react quickly, warn others, and communicate. An example of one such system is Live Alarm by Lifeline Applications. Live Alarm is a mobile and web first responder app designed to help first responders, workplace safety, school security, and church security respond to a variety of emergencies including an active shooter or mass shooting situation. To learn more, please visit the Lifeline Applications website.

Where can I find more resources on this topic?

You can request a free list of reputable resources here. We regularly review new material to keep our holdings current.

If you have questions or comments about this article, please contact us.



[i] Comment: The author is indebted to the work of Rosanna Smart, Terry L. Schell, and RAND in their highly valuable Mass Shootings in the United States, a key source for this article. Citation: Rosanna Smart and Terry L. Schell, "Mass Shootings in the United States," RAND Corporation, April 15, 2021.

[ii] Federal Bureau of Investigation (FBI): Active Shooter Incidents in the United States in 2020.

[iii] Federal Bureau of Investigation (FBI): Active Shooter Incidents 20-Year Review, 2000-2019.

[iv] Federal Bureau of Investigation (FBI): Active Shooter Incidents in the United States in 2019.

[v] Federal Bureau of Investigation (FBI): Active Shooter Incidents in the United States from 2000-2018.

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