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A lone wolf attack, or lone actor attack, is a particular kind of mass murder, committed in a public setting by an individual who plans and commits the act on their own. In the United States, such attacks are usually committed with firearms. In other countries, knives are sometimes used to commit mass stabbings. Although definitions vary, most databases require a minimum of four victims (including injured) for the event to be considered a mass murder.
Lone actor attacks have become the subject of academic research. Studies have found that some lone actor attacks are committed because of personal grievances and a desire for revenge, while others are acts of terrorism, intended to induce fear and influence the way people think.
The academic definition of lone actor mass shootings means they occur in a public setting and excludes the killing of multiple people if those deaths occur during the commission of other crimes, such as bank robberies or during gang warfare. The definition also excludes killings such as familicide, where the perpetrator kills the rest of their family in a private setting. Criminologist Grant Duwe identified 845 mass shootings in the United States between 1976 and 2018. However, only 158 of these met the criteria for a lone actor shooting which occurred in a public setting.
The descriptor 'lone wolf' is derived from the notion of a lone wolf, a pack animal that has left or been excluded from its pack. This particular term is more likely to be used by American law enforcement than by academics who study this phenomenon.