The Washington Post: 4+ shot and killed in one incident, excluding the perpetrator(s), at a public place, excluding gang-related killings.

Mother Jones: 3+ shot and killed in one incident, excluding the perpetrator(s), at a public place, excluding gang-related killings.

Gun Violence Archive/ Vox: 4+ shot in one incident, excluding the perpetrator(s), at one location, at roughly the same time.

Mass Shooting Tracker: 4+ shot in one incident, at one location, at roughly the same time.

Stanford MSA Data Project: 3+ shot in one incident, at one location, at roughly the same time, excluding organized crime, as well as gang-related and drug-related shootings.

Listed roughly from most broad to most restrictive: There are varying definitions of a mass shooting.

3 List of mass shootings (20th century).2 List of mass shootings (21st century).Larger documentation of mass shootings in the United States has occurred through independent and scholarly studies such as the Stanford University Mass Shootings in America Data Project. The crowdsourced Mass Shooting Tracker project uses a looser definition than the Gun Violence Archive's definition: four people shot in one incident regardless of the circumstances. The Washington Post and Mother Jones use similar definitions, with the latter acknowledging that their definition 'is a conservative measure of the problem', as many rampages with fewer fatalities occur. The Congressional Research Service narrows that definition further, only considering what it defines as 'public mass shootings', and only considering victims as those who are killed, excluding any victims who survive. The Gun Violence Archive, a nonprofit research group that tracks shootings and their characteristics in the United States, defines a mass shooting as an incident in which four or more people, excluding the perpetrator(s), are shot in one location at roughly the same time. The precise inclusion criteria are disputed, and there is no broadly accepted definition. Mass shootings are incidents involving several victims of firearm-related violence. This is a list of the most notable mass shootings in the United States that have occurred since 1920.

Class=notpageimage| Mass shootings in the Northern Mariana Islands

The gone home incident august 13 2010