因美国宪法第二修正案规定保障美国公民的持枪自由。自此在美国,公民不仅可以合法持有枪支,而且取得方式也相对容易。但同时也造成美国是世界上枪支暴力问题最严重的国家之一。目前,美國政府沒有對 “大规模枪击事件”採用固定的定义[8]。不同的机构对其定义不同。如2003年1月,美国国会通过了一项有关暴力犯罪的法案(The Investigative Assistance for Violent Crimes Act of 2012),该法案定义“大规模杀害事件”为除凶手自身以外,至少三人死亡的事件[9][10][11]。2015年,美国国会研究服务定义大规模枪击事件为“在事件發生及其相邻区域内,至少四名受害者被枪击杀害的多重杀人犯罪”[12]。非营利组织“枪支暴力档案”所使用的宽泛定义则是“除凶手自身以外,至少四人被枪击(无论死亡与否)的事件”[13],而此定义也往往为新闻界和非盈利组织所采用[14][15][16][17][18]。
^Greenberg, Jacobson & Valverde, Jon, Louis & Miriam. What we know about mass shootings. Tampa Bay Times. February 14, 2018 [20 February 2018]. (原始内容存档于2019-09-25). As noted above, there is no widely accepted definition of mass shootings. People use either broad or restrictive definitions of mass shootings to reinforce their stance on gun control. After the 2012 Sandy Hook school shooting, Congress defined "mass killings" as three or more homicides in a single incident. The definition was intended to clarify when the U.S. Attorney General could assist state and local authorities in investigations of violent acts and shootings in places of public use.
^Ingraham, Christopher. What makes a 'mass shooting' in America. Washington Post. 3 December 2015 [5 November 2017]. (原始内容存档于2021-04-07). But starting in 2013, federal statutes defined "mass killing" as three or more people killed, regardless of weapons.
^Follman, Mark. What Exactly Is A Mass Shooting. Mother Jones. [August 9, 2015]. (原始内容存档于2017-05-15). In January 2013, a mandate for federal investigation of mass shootings authorized by President Barack Obama lowered that baseline to three or more victims killed.