This is a list of notable mass shootings in the United States in the 20th century.
Mass shootings are incidents involving several victims of firearm-related violence.
The precise inclusion criteria are disputed, and there is no broadly accepted definition.
[1][2] 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.
Detailed lists of shootings can be found per-year at their respective pages.