A mass shooting is an incident involving multiple victims of firearms-related violence. The United States' Congressional Research Service acknowledges that there is not a broadly accepted definition, and defines a "public mass shooting" as one in which four or more people selected indiscriminately, not including the perpetrator, are killed, echoing the FBI definition of the term "mass murder". However, according to the Investigative Assistance for Violent Crimes Act of 2012, signed into law in January 2013, a mass killing is defined as a killing with at least three deaths, excluding the perpetrator. Another unofficial definition of a mass shooting is an event involving the shooting (not necessarily resulting in death) of five or more people (sometimes four) with no cooling-off period.  Related terms include school shooting and massacre.
A mass shooting may be committed by individuals or organizations in public or non-public places. Terrorist groups in recent times have used the tactic of mass shootings to fulfill their political aims. Individuals who commit mass shootings may fall into any of a number of categories, including killers of family, of coworkers, of students, and of random strangers. Individuals' motives for shooting vary.
Responses to mass shootings take a variety of forms, depending on the context: number of casualties, the country, political climate, and other factors. The media cover mass shootings extensively, and, often, sensationally, and the effect of that coverage has been examined. Countries such as the United Kingdom and Australia have changed their gun laws in the wake of mass shootings. In contrast, the United States' constitution prohibits laws which disallow firearm ownership outright and owns about half of the world's guns.
The characterization of an event as a mass shooting depends upon definition and definitions vary. Under U.S. federal law the Attorney General may on a request from a state assist in investigating "mass killings", rather than mass shootings. The term was originally defined as the murder of four or more people with no cooling-off period but redefined by Congress in 2013 as being murder of three or more people. According to CNN, a mass shooting is defined as having four or more fatalities, not including gang killings or slayings that involve the death of multiple family members. In "Behind the Bloodshed", a report by USAToday, a mass killing is defined as any incident in which four or more were killed and also includes family killings. A crowdsourced data site cited by CNN, MSNBC, The New York Times, The Washington Post, The Economist, the BBC, etc., Mass Shooting Tracker, defines a mass shooting as any incident in which four or more people are shot, whether injured or killed. As of November 2017, the FBI defines a mass shooting as an incident involving "four or more people shot at once." A noteworthy connection has been reported in the U.S. between mass shootings and domestic or family violence, with a current or former intimate partner or family member killed in 76 of 133 cases (57%), and a perpetrator having previously been charged with domestic violence in 21. The lack of a single definition can lead to alarmism in the news media, with some reports conflating categories of crimes.
In Australia, a 2006 paper defined a mass shooting as "one in which ⩾5 firearm‐related homicides are committed by one or two perpetrators in proximate events in a civilian setting, not counting any perpetrators".
Crime violence research group Gun Violence Archive, whose research is used by all major American media outlets defines Mass Shooting as "FOUR or more shot and/or killed in a single event [incident], at the same general time and location not including the shooter" differentiating between Mass Shooting and Mass Murder [Killing] and not counting shooters as victims.
An act is typically defined as terrorist if it "appears to have been intended" to intimidate or to coerce people; a mass shooting is not, in itself, an act of terrorism. A U.S. congressional research service report explicitly excluded from its definition of public mass shootings those in which the violence is a means to an end, for example where the gunmen "pursue criminal profit or kill in the name of terrorist ideologies".
Mass shootings have occurred on the African continent, including the 2015 Sousse attacks, the 2015 Bamako hotel attack, the 2013 Westgate shopping mall attack in Nairobi, Kenya, and the 1994 Kampala wedding massacre. Most mass shootings in Africa have stemmed from terrorism, with tourists and diplomats frequently being the targets. Workplace violence and prejudice against ethnic minorities have less-frequently been involved in such spontaneous acts of mass violence.
The single deadliest event was the 2008 Mumbai attacks in which 164 people were killed and a further 308 people were wounded by terrorists.
South Korea has suffered multiple mass shootings in the South Korean Army, mainly due to soldier's stress and conflicts from its violence and detention from society.
Japan has as few as two gun-related homicides per year. These numbers include all homicides in the country, not just mass shootings.
There have been many mass shootings in Israel such as the 1972 Lod Airport Massacre, which killed 26 and injured 80, the 2002 Bat Mitzvah massacre and the June 2016, massacre at the popular Sarona center complex. These were all planned or executed by Palestinian or Arab terrorists.
In addition there have been two mass shootings by Jews in Israel. In 1991, Ami Popper was convicted of murdering seven Palestinian men in a mass shooting carried out in 1990. In 1994 Baruch Goldstein murdered 29 Muslims worshipping and injuring a further 125 in Hebron. Also known as the Cave of the Patriarchs massacre.
Several mass shootings have occurred in Europe, including the November 2015 Paris attacks, the 2012 Toulouse and Montauban shootings, the 2011 Norway attacks, the 2009 Winnenden school shooting, the 2007 Jokela school shooting, the 2008 Kauhajoki school shooting, the 2001 Zug massacre, the 2002 Erfurt massacre, the 1987 Hungerford massacre, the 1990 Puerto Hurraco massacre, the 1993 Greysteel massacre, the 2010 Cumbria shootings and the 1996 Dunblane massacre.
Notable mass shootings include the 1992 Tatarstan shooting, the 2002 Yaroslavsky shooting, the 2002 Moscow theater hostage crisis, the 2004 Beslan school siege, the 2012 Moscow shooting, the 2013 Belgorod shooting, and the 2014 Moscow school shooting.
Notable mass shootings in Canada include the 1989 École Polytechnique massacre, the 1992 Concordia University massacre, the 2012 Danzig Street shooting, the 2014 Edmonton killings and the 2017 Quebec City mosque shooting.
In one study by Adam Lankford, it has been estimated that 31% of public mass shootings occur in the U.S., although it has only 5% of the world's population. CNN cites a study by criminologist A. Lankford that finds that "there are more public mass shootings in the United States than in any other country in the world". The study concludes that "The United States and other nations with high firearm ownership rates may be particularly susceptible to future public mass shootings, even if they are relatively peaceful or mentally healthy according to other national indicators." Criminologist Gary Kleck criticized Adam's findings stating the study fails to provide evidence that gun ownership increases mass shootings and that Lankford has been unwilling to share a list of his cases, provide a list of the number of attacks per country, or even list his sources so that others can check his numbers. Mass shootings have also been observed to be followed by an increase in the purchase of weapons, but this phenomenon seems to be driven by a minority since neither gun owners nor non-owners report an increased feeling of needing guns.
After mass shootings, some survivors have written about their experiences and their experiences have been covered by journalists. A survivor of the Knoxville Unitarian Universalist church shooting wrote about his reaction to other mass shooting incidents. The father of a victim in a mass shooting at a movie theater in Aurora, Colorado, wrote about witnessing other mass shootings after the loss of his son. The survivors of the 2011 Norway attacks recounted their experience to GQ. In addition, one paper studied Swedish police officers' reactions to a mass shooting.
Notable mass shooters from outside the United States include Anders Behring Breivik (Norway, 2011), Tim Kretschmer (Germany, 2009), William Unek (Africa, 1954 and 1957), Marc Lépine and Valery Fabrikant, (Canada, 1989 and 1992), Pekka-Eric Auvinen and Matti Juhani Saari (Finland, 2007 and 2008), Richard Komakech (Uganda, 1994), Omar Abdul Razeq Abdullah Rifai (Egypt, 2013), Farda Gadirov (Azerbaijan, 2009), Martin Bryant (Australia, 1996), Michael Robert Ryan and Derrick Bird (England, 1987 and 2010), Thomas Hamilton (Scotland, 1996) and Woo Bum-kon (South Korea, 1982).
Notable perpetrators of massacres in the U.S. include Edward Charles Allaway, James Edward Pough, Carl Robert Brown, Omar Mateen, Robert A. Hawkins, James Oliver Huberty, Nathan Dunlap, George Hennard, Dylann Roof, Adam Lanza, Nidal Malik Hasan, Charles Whitman, Jeff Weise, Gang Lu, Patrick Sherrill, Barry Loukaitis, Esteban Santiago, Christopher Harper-Mercer, Gian Luigi Ferri, Mark Essex, Scott Evans Dekraai, Steven Kazmierczak, Jennifer San Marco, James Eagan Holmes, Michael McLendon, Jared Lee Loughner, Seung-Hui Cho, Elliot Rodger, Charles Carl Roberts IV, Rizwan Farook and Tashfeen Malik, Robert Lewis Dear, Mitchell Johnson and Andrew Golden, Aaron Alexis, Wade Michael Page, Eric Harris and Dylan Klebold, Patrick Edward Purdy, Gavin Eugene Long, Micah Xavier Johnson, Kyle Aaron Huff, One L. Goh, Stephen Paddock, Devin Patrick Kelley and Nikolas Cruz. U.S. mass shooters are overwhelmingly males. According to a database compiled by Mother Jones magazine, the race of the shooters is approximately proportionate to the overall U.S. population, although Asians are overrepresented and Latinos underrepresented. Criminologist James Allen Fox said that most mass murderers do not have a criminal record, or involuntary incarceration at a mental health center, but an article in the New York Times in December 2015 about 15 recent mass shootings found that six perpetrators had had run-ins with law enforcement, and six had mental health issues.
Mass shootings can be motivated by misanthropy and terrorism and caused by mental illness, inceldom and extensive bullying among other reasons. Forensic psychologist Stephen Ross says that extreme anger and the thought shooters are working for a cause, rather than mental illness, is most often the explanation. A study by Vanderbilt University researchers found that "fewer than 5% of the 120,000 gun-related killings in the United States between 2001 and 2010 were perpetrated by people diagnosed with mental illness". John Roman of the Urban Institute argues that, while better access to mental health care, restricting high powered weapons, and creating a defensive infrastructure to combat terrorism are constructive, they don't address the greater issue, which is "we have a lot of really angry young men in our country and in the world."
Author Dave Cullen described killer Eric Harris as an "injustice collector" in his 2009 book Columbine. He expanded on the concept in a 2015 New Republic essay on injustice collectors, identifying several notorious killers as fitting the category, including Christopher Dorner, Elliot Rodger, Vester Flanagan, and Andrew Kehoe. Likewise, mass shooting expert and former FBI profiler Mary O'Toole also uses the phrase "injustice collector" in characterizing motives of some mass shooting perpetrators. In relation, criminologist James Alan Fox contends that mass murderers are "enabled by social isolation" and typically experience "years of disappointment and failure that produce a mix of profound hopelessness and deep-seated resentment." Jillian Peterson, an assistant professor of criminology at Hamline University who is participating in the construction of a database on mass shooters, noted that two phenomena surface repeatedly in the statistics: hopelessness and a need for notoriety in life or in death. Notoriety was first suggested as a possible motive and researched by Justin Nutt. Nutt stated in a 2013 article, "those who feel nameless and as though no one will care or remember them when they are gone may feel doing something such as a school shooting will make sure they are remembered and listed in the history books." 
In considering the frequency of mass shootings in the United States, criminologist Peter Squires says that the individualistic culture in the United States puts the country at greater risk for mass shootings than other countries, noting that "many other countries where gun ownership is high, such as Norway, Finland, Switzerland and Israel . . . tend to have more tight-knit societies where a strong social bond supports people through crises, and mass killings are fewer." He is an advocate of gun control, but contends there is more to mass shootings than the prevalence of guns.
According to Michael Cook and Carolyn Moynihan of Mercatornet, an angle that is missed by mainstream media is the findings of important social scientists such as eminent Harvard sociologist Robert J. Sampson who wrote: “Family structure is one of the strongest, if not the strongest, predictor of variations in urban violence across cities in the United States. The close empirical connection between family breakdown and crime suggests that increased spending on crime-fighting, imprisonment, and criminal justice in the United States over the last 40 years is largely the direct or indirect consequence of marital breakdown.” His views are echoed by the eminent criminologists Michael Gottfredson and Travis Hirschi, who have written that “such family measures as the percentage of the population divorced, the percentage of households headed by women, and the percentage of unattached individuals in the community are among the most powerful predictors of crime rates.”
Based on the research of another social scientist who was himself raised by a single mother, Bradford Wilcox, "boys living in single mother homes are almost twice as likely to end up delinquent compared to boys who enjoy good relationships with their father."
Moynihan said that "almost all school shooters come from families where the parents are either divorced or alienated", and Cook argued that "perhaps they wouldn’t need more gun control if they had better divorce control.”
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Some people have considered whether media attention revolving around the perpetrators of mass shootings is a factor in sparking further incidents. In response to this, some in law enforcement have decided against naming mass shooting suspects in media-related events, in order to avoid giving them notoriety.
The effects of messages used in the coverage of mass shootings has been studied. Researchers studied the role the coverage plays in shaping attitudes toward persons with serious mental illness and public support for gun control policies.
Some news media have weighed in on the gun control debate. After the 2015 San Bernardino attack, the New York Daily News' front-page headline, "God isn't fixing this", was accompanied by "images of tweets from leading Republicans who shared their 'thoughts' and 'prayers' for the shooting victims".
Responses to mass shootings take a variety of forms, depending on the country and political climate.
After the 1996 Port Arthur massacre in Australia, the government changed gun laws in Australia. As in the United States, figures vary according to the definition of "mass shooting"; a 2006 paper used a definition "one in which ⩾5 firearm‐related homicides are committed by one or two perpetrators in proximate events in a civilian setting, not counting any perpetrators", compared to the usual U.S. definition of an indiscriminate rampage in public places resulting in four or more victims killed. Between 1981 and the passing of the law in 1996 there were 13 mass shootings with five or more deaths; in the following decade, while the new law was in place, there were no such mass shootings. Overall gun deaths have continued to decline for two decades since the law was passed.
There were four significant shootings, though not meeting the "mass shooting" definition of the 2006 paper, between 1996 and June 2016[update]:
As a result of the 1987 Hungerford massacre and 1996 Dunblane school massacre mass shootings, the United Kingdom enacted tough gun laws and a buyback program to remove guns from private ownership. There has been one mass shooting since the laws were restricted, the Cumbria shootings in 2010 which killed 13 people.
In the United States, support for gun law reform varies considerably by political party, with Democrats generally more supportive and Republicans generally more opposed. Some in the U.S. believe that tightening gun laws would prevent future mass shootings. Some politicians in the U.S. introduced legislation to reform the background check system for purchasing a gun. A vast majority of Americans support tighter background checks. "According to a poll [...] by Quinnipiac University in Connecticut, 93 percent of registered voters said they would support universal background checks for all gun buyers."
Others contend that mass shootings should not be the main focus in the gun law reform debate because these shootings account for less than one percent of the U.S. homicide rate and believe that these shootings are hard to stop. They often argue that civilians with concealed guns will be able to stop shootings.
Gun control policies may cause a lot of controversy due to divided opinions on who should be able to carry a weapon. An opinion survey was conducted by the firm GfK Knowledge Networks to differentiate between the different attitudes towards gun control. There was a gun policy survey and a mental illness survey. Studies showed that over 85% of those questioned supported national background checks into the mental health records of citizens attempting to purchase a gun. More than 50% of people felt that those suffering with mental health issues were more deviant and threatening than those who had good mental health. The study also proved that there is large interest in contributing to mental health awareness as well as simply prohibiting those suffering from purchasing guns. Nearly two thirds of respondents supported greater government spending on mental health, with more than 60% of people believing this would reduce gun violence in the USA. (Colleen L. Barry, 2013)
As of June 2016, U.S. President Barack Obama had spoken in the aftermath of fourteen mass shootings during his nearly eight-year presidency, repeatedly calling for more gun safety laws in the United States. After the Charleston church shooting, U.S. President Barack Obama said, "At some point, we as a country will have to reckon with the fact that this type of mass violence does not happen in other advanced countries. It doesn't happen in other places with this kind of frequency." After the December 2015 San Bernardino attack, Obama renewed his call for reforming gun-safety laws and also said that the frequency of mass shootings in the United States has "no parallel in the world". After the February 2018 attack at Florida's Parkland school shooting at Stoneman Douglas High School, the school's student survivors, teachers, and parents became strong leaders in the effort to ban assault weapon sales and easy accessibility to military weapons.
(I)the term mass killings means 3 or more killings in a single incident;
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.
But starting in 2013, federal statutes defined "mass killing" as three or more people killed, regardless of weapons.
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.