Mass shootings are attacks in which one or more people use firearms to kill or attempt to kill multiple people in a single incident. The term is widely used in media, research, policing and politics, but there is no single worldwide definition.
Definitions vary by country and by data set. Some systems count only deaths, some count deaths and injuries, some include domestic incidents, and some focus on public attacks. The FBI uses the related term active shooter for one or more people actively engaged in killing or attempting to kill people in a populated area. United States law also uses the term mass killing for incidents with three or more killings in a single incident.
Definition
The simplest meaning is a shooting with multiple victims. The difficulty is deciding how many victims are required, whether the attacker is counted, whether injuries are counted, and whether the incident must happen in a public place.
Researchers often separate several overlapping categories:
- public mass shootings, such as attacks in schools, workplaces, places of worship, entertainment venues or public streets
- family or domestic mass killings
- gang, organised-crime or dispute-related shootings
- terrorist attacks involving firearms
- active shooter incidents where police and emergency services must respond to an ongoing threat
These categories matter because the motives, warning signs, weapons, targets and prevention options can differ. A single headline category can hide important differences between incidents.
Causes and Risk Factors
Mass shootings do not have one simple cause. Common factors include grievance, fixation on previous attacks, suicidal intent, access to firearms, leakage of threats, domestic abuse, extremist ideology, workplace conflict, school conflict, fame-seeking, and severe personal crisis.
Mental health is often discussed after mass shootings, but it should be handled carefully. Some attackers have had mental health problems, but most people with mental illness are not violent. Treating mental illness as the main explanation can distort the issue and stigmatise people who are already more likely to need support than suspicion.
Access to firearms affects the scale and speed of an attack. High-capacity weapons, spare ammunition, poor storage and easy access can increase lethality. At the same time, weapon access is not the only factor. Planning, target choice, emergency response, warning signs and social context also matter.
Copycat and Contagion Effects
Some attackers study previous mass killings. They may imitate methods, dates, clothing, writings, livestreaming, manifesto language or target choices. This is why many journalists, researchers and prevention groups recommend limiting repeated use of attackers' names and images, avoiding unnecessary detail about methods, and focusing more on victims, survivors and prevention.
Online spaces can amplify this problem. Attackers may seek status in extremist or nihilistic communities, trade old attack material, or try to create content designed for spread after death or arrest. Moderation, reporting pathways and rapid removal of attack footage can reduce the reach of that material.
Notable Incidents
Mass shootings occur in many countries, although patterns differ by law, culture and weapon availability. Widely discussed examples include:
- the Columbine High School attack in Colorado in 1999, carried out by Eric Harris and Dylan Klebold
- the Sandy Hook Elementary School shooting in Connecticut in 2012, carried out by Adam Lanza
- the Las Vegas shooting in Nevada in 2017, carried out by Stephen Paddock
- the Christchurch mosque attacks in New Zealand in 2019, carried out by Brenton Tarrant
Examples should not be treated as a ranking or a script. The main value of studying these cases is to understand warning signs, emergency response, survivor needs, policy failures and ways to reduce future harm.
Prevention and Response
Prevention usually works best when it combines several approaches. Threat assessment teams can act when a person leaks intent, makes threats, fixates on violence or shows escalating grievance. Secure firearm storage can reduce access by people in crisis. Domestic abuse intervention can matter because a number of mass killings begin in a family or intimate-partner setting.
Emergency response has also changed. Schools, workplaces and public venues often use lockdown procedures, evacuation plans, communication systems and trauma training. Police tactics in active shooter incidents have shifted towards rapid engagement of the attacker when the threat is ongoing.
After an attack, communities need medical care, mental health support, victim services, accurate information and long-term help. The effects can last for years, including injuries, bereavement, post-traumatic stress, disability, litigation, memorial disputes and policy arguments.
Public Debate
Mass shootings often lead to debate about firearms law, policing, school security, mental health services, online extremism, media coverage, domestic violence, and civil liberties. These debates can become highly polarised, especially in the United States.
Good analysis separates evidence from slogans. It asks what type of incident is being discussed, which interventions have evidence behind them, which rights are affected, and whether proposed changes would have applied to the facts of the attack.
See Also
References
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