Mass shootings in the United States

Mass shootings are incidents involving multiple victims of firearm-related violence. The precise inclusion criteria are disputed, and there is no broadly accepted definition. One definition is an act of public firearm violence—excluding gang killings, domestic violence, or terrorist acts sponsored by an organization—in which a shooter kills at least four victims. Using this definition, one study found that nearly one-third of the world's public mass shootings between 1966 and 2012 (90 of 292 incidents) occurred in the United States. Using a similar definition, "The Washington Post" records 163 mass shootings in the United States between 1967 and June 2019.
Gun Violence Archive, frequently cited by the press, defines a mass shooting as firearm violence resulting in at least four people being shot at roughly the same time and location, excluding the perpetrator. Using this definition, there have been 2,128 mass shootings since 2013, roughly one per day.
According to some studies, the United States has had more mass shootings than any other country, however they accounted for less than two-tenths of 1% (0.2%) of homicides between 2000 and 2016. Shooters generally either die by suicide afterwards or are restrained or killed by law enforcement officers or civilians. 
Definitions.
There is no fixed definition of a mass shooting in the United States. The Investigative Assistance for Violent Crimes Act of 2012, signed into law in January 2013, defines a "mass killing" as one resulting in at least 3 victims, excluding the perpetrator. In 2015, the Congressional Research Service does not define a mass shooting but does define a public mass shooting—for the purposes of its report entitled "Mass Murder with Firearms"—as "a multiple homicide incident in which four or more victims are murdered with firearms, within one event, and in one or more locations in close proximity". The CRS further states that its report "attempts to refine the relatively broad concept of mass shooting [...] into a narrower formulation: public mass shootings." A broader definition, as used by the Gun Violence Archive, is that of "4 or more shot or killed, not including the shooter". This definition, of four people shot regardless of whether or not that results in injury or death, is often used by the media, press, and non-profit organizations.
Frequency.
Some studies indicate that the rate at which public mass shootings occur has tripled since 2011. Between 1982 and 2011, a mass shooting occurred roughly once every 200 days. However, between 2011 and 2014, that rate has accelerated greatly with at least one mass shooting occurring every 64 days in the United States.
In recent years, the number of public mass shootings has increased substantially, although there has been an approximately 50% decrease in firearm homicides in the nation overall since 1993. The decrease in firearm homicides has been attributed to better policing, a better economy and environmental factors such as the removal of lead from gasoline.
Differing sources.
A comprehensive report by "USA Today" tracked all mass killings from 2006 through 2017 in which the perpetrator willfully killed 4 or more people. For mass killings by firearm for instance, it found 271 incidents with a total of 1,358 victims. "Mother Jones" listed seven mass shootings, defined as indiscriminate rampages in public places resulting in four or more victims killed, in the U.S. for 2015. An analysis by Michael Bloomberg's gun violence prevention group, "Everytown for Gun Safety", identified 110 mass shootings, defined as shootings in which at least four people were murdered with a firearm, between January 2009 and July 2014; at least 57% were related to domestic or family violence.
Other media outlets have reported that hundreds of mass shootings take place in the United States in a single calendar year, citing a crowd-funded website known as "Shooting Tracker" which defines a mass shooting as having four or more people injured or killed. In December 2015, "The Washington Post" reported that there had been 355 mass shootings in the United States so far that year. In August 2015, "The Washington Post" reported that the United States was averaging one mass shooting per day. An earlier report had indicated that in 2015 alone, there had been 294 mass shootings that killed or injured 1,464 people. "Shooting Tracker" and "Mass Shooting Tracker", the two sites that the media have been citing, have been criticized for using a broader criteria—counting four victims injured as a mass shooting—thus producing much higher figures.
Mass shootings tend to occur in clusters. When one occurs another is likely to follow, according to research by the Violence Project.
Demographics.
According to "The New York Times", the majority of perpetrators they have published stories about are white males who act alone. According to most analyses and studies, the proportion of mass shooters in the United States who are white is slightly less than the proportion of white people in the general population of the US, however, the proportion of male mass shooters is considerably greater than the proportion of males in the population. According to the Associated Press, white men comprise nearly 50 percent of all mass shooters in the US. According to the Center for Inquiry, mass shootings of family members (the most common) are usually carried out by White, middle-aged men. Felony mass shootings (connected with a previous crime) tend to be committed by young Black or Hispanic males with extensive criminal records, typically against persons of the same ethnic group. Public mass shootings of persons unrelated to the shooter, and for a reason not connected with a previous crime (the rarest but most publicized) are committed by men whose racial distribution closely matches that of the nation as a whole. Other than gender, the demographic profiles of public mass shooters are too varied to draw firm conclusions.
Contributing factors.
Several possible factors may work together to create a fertile environment for mass murder in the United States. Most commonly suggested include:
A panel of mental health and law enforcement experts has estimated that roughly one-third of acts of mass violence—defined as crimes in which four or more people were killed—since the 1990s were committed by people with a serious mental illness. However, the study emphasized that people with serious mental illness are responsible for less than 4% of all the violent acts committed in the United States.
Weapons used.
Several types of guns have been used in mass shootings in the United States. A 2014 study conducted by Dr. James Fox of 142 shootings found that 88 (62%) were committed with handguns of all types; 68 (48%) with semi-automatic handguns, 20 (14%) with revolvers, 35 (25%) with semi-automatic rifles, and 19 (13%) with shotguns. The study was conducted using the "Mother Jones" database of mass shootings from 1982 to 2018. High capacity magazines were used in approximately half of mass shootings. Semi-automatic rifles have been used in six of the ten deadliest mass shooting events.
Deadliest mass shootings since 1949.
The following mass shootings are the deadliest to have occurred in modern U.S. history. Only incidents with ten or more civilian fatalities are included. This list starts in 1949, the year in which Howard Unruh committed his shooting, the first to incur ten or more fatalities. The death tolls reflect the numbers given by police in the days after the shootings.