What is a mass shooting?
Broadly speaking, the term refers to an incident involving multiple victims of gun violence. But there is no official set of criteria or definition for a mass shooting, according to criminology experts and FBI officials who have spoken with Mother Jones.
Generally, there are three terms you'll see to describe a perpetrator of this type of gun violence: mass murderer, spree killer, or serial killer. An FBI crime classification report from 2005 identifies an individual as a mass murderer if he kills four or more people in a single incident (not including himself), typically in a single location. (The baseline of four fatalities is key—more on that just below.)
The primary distinction between a mass murderer and a spree killer is that the latter strikes in multiple locations, though still in a relatively short time frame. A serial killer is distinguished by striking over a longer time frame, in multiple locations, with opportunity for what the FBI report refers to as "cooling-off periods" in between attacks.
How often do mass shootings occur?
Beginning in July, after the movie theater slaughter in Aurora, Colorado, we analyzed and mapped 62 mass shootings from the last 30 years. As we delved into the research, we realized that robust data on this subject was hard to come by, in part due to the lack of clear criteria. We were focused on the question of how many times Aurora-like events—disturbingly all too frequent—had actually happened. We honed our criteria accordingly:
■ We excluded crimes involving armed robbery or gang violence;
■ The attack must have occurred in essentially a single incident, in a public place;
■ The killer, in accordance with the FBI guideline, had to have taken the lives of at least four people.
The traumatic events included in our guide to mass shootings are the kind that tend to grab national attention—school and workplace shootings, attacks in shopping malls or government buildings—but they represent only a sliver of America's gun violence.