Any mass shootings and subsequent calls to amend gun control laws will likely generate requests for comprehensive data on the prevalence and deadliness of these incidents. To these ends, this report provides data and analysis on mass shootings, that is, mass murders committed entirely with firearms, for a 15-year period (1999-2013) and mass public shootings for the 44-year period (1970-2013). In the wake of tragedy in 2012 in Newtown, CT, Congress defined "mass killings" to mean "3 or more killings in a single incident" ( P.L. 112-265 ; January 14, 2013). A handful of criminologists, statisticians, sociologists, and other researchers have analyzed the principal source of national homicide statistics that is compiled by the Department of Justice (DOJ) annually, as part of the FBI's Uniform Crime Reports and Supplementary Homicide Reports (UCR-SHR). Drawing on the work of James Alan Fox and Jack Levin, Grant Duwe, and Meghan Hoyer (and colleagues at USA Today ), CRS took the following steps:
analyzed the FBI SHR data, the nation's primary data source on murder and nonnegligent manslaughter in the United States; verified the mass murders reported to the FBI by checking press accounts and, when needed, consulted with the reporting police agencies themselves; cross-referenced this data with mass murders with firearms lists compiled by advocacy groups, media outlets, and law enforcement agencies; supplemented the SHR data with mass shootings reported in the press, but not reported to the FBI or previously compiled by other researchers; evaluated every incident based on victim-offender relationships, incident locations, and other pertinent event characteristics and circumstances; and found three broad patterns of mass shootings that could provide policymakers with improved vantage points from which to evaluate gun control proposals. Based on FBI guidance in part, Duwe, and others, CRS adopted the following parallel definitions for patterns of "mass murder" committed entirely with firearms:
"mass shooting" means a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and in one or more locations in close geographical proximity; "mass public shooting" means a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and at least some of the murders occurred in a public location or locations in close geographical proximity (e.g., a workplace, school, restaurant, or other public settings), and the murders are not attributable to any other underlying criminal activity or commonplace circumstance (armed robbery, criminal competition, insurance fraud, argument, or romantic triangle); "familicide mass shooting" means a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and a majority of the victims were members of the offender's immediate or extended family, the majority of whom were murdered in one or more private residences or secluded, sparsely populated settings in close geographical proximity, and the murders are not attributable to any other underlying criminal activity or commonplace circumstance (e.g., armed robbery, criminal competition, insurance fraud, argument, or romantic triangle); and "other felony mass shooting" means a multiple victim homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, in one or more locations in close geographical proximity, and the murders are attributable to some other underlying criminal activity or commonplace circumstance (e.g., armed robbery, criminal competition, insurance fraud, argument, or romantic triangle). Mass Shootings Findings

As shown in Figure 1 , CRS analysis of the FBI SHR and other data sources indicates that offenders committed at least 317 mass shooting incidents in the United States, murdering 1,554 victims and non-fatally wounding another 441 victims, from 1999 through 2013. The 15-year dataset compiled by CRS indicates that the prevalence and deadliness of mass shootings overall fluctuated sporadically from year to year. Based on the CRS definition of "mass public shootings," the data show there were on average:
one (1.1) incident per year during the 1970s (5.5 victims murdered, 2.0 wounded per incident), nearly three (2.7) incidents per year during the 1980s (6.1 victims murdered, 5.3 wounded per incident), four (4.0) incidents per year during the 1990s (5.6 victims murdered, 5.5 wounded per incident), four (4.1) incidents per year during the 2000s (6.4 victims murdered, 4.0 wounded per incident), and four (4.5) incidents per year from 2010 through 2013 (7.4 victims murdered, 6.3 wounded per incident). These decade-long averages indicate that the prevalence, if not the deadliness, of mass public shootings has increased, but whether these increases constituted an "epidemic," as some have argued, would be a matter of perspective. The first four years of this decade saw an uptick in both the prevalence and deadliness of those incidents. Building upon Duwe's data and analysis, CRS compiled a 44-year dataset of firearms-related mass murders that could arguably be characterized as "mass public shootings." Possible Issues and Options for Congress

Mass shootings are arguably one of the worst manifestations of gun violence. Did the offenders have a history of violence and/or mental illness? Did the gun types lead to higher victim counts in terms of both killed and wounded?
In the wake of tragedy in Newtown, CT, Congress defined "mass killings" as "3 or more killings in a single incident" (P.L. 112-265). Any consideration of new or existing gun laws that follows mass shootings is likely to generate requests for comprehensive data on the prevalence and deadliness of these incidents. Despite the pathos of mass shootings, only a handful of researchers and journalists have analyzed the principal source of homicide data in the United States—the Supplementary Homicide Reports (SHR) compiled by the Federal Bureau of Investigation (FBI)—to determine whether those incidents have become more prevalent and deadly.
According to the FBI, the term "mass murder" has been defined generally as a multiple homicide incident in which four or more victims are murdered, within one event, and in one or more locations in close geographical proximity. Based on this definition, for the purposes of this report, "mass shooting" is defined 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. Similarly, a "mass public shooting" is defined to mean a multiple homicide incident in which four or more victims are murdered with firearms, within one event, in at least one or more public locations, such as a workplace, school, restaurant, house of worship, neighborhood, or other public setting.
This report analyzes mass shootings for a 15-year period (1999-2013). CRS analysis of the FBI SHR dataset and other research indicates that offenders committed at least 317 mass shootings, murdered 1,554 victims, and nonfatally wounded another 441 victims entirely with firearms during that 15-year period. The prevalence of mass shooting incidents and victim counts fluctuated sporadically from year to year. For the period 2007-2013, the annual averages for both incidents and victim counts were slightly higher than the years from 1999-2007.
With data provided by criminologist Grant Duwe, CRS also compiled a 44-year (1970-2013) dataset of firearms-related mass murders that could arguably be characterized as "mass public shootings." These data show that there were on average:
one (1.1) incident per year during the 1970s (5.5 victims murdered, 2.0 wounded per incident), nearly three (2.7) incidents per year during the 1980s (6.1 victims murdered, 5.3 wounded per incident), four (4.0) incidents per year during the 1990s (5.6 victims murdered, 5.5 wounded per incident), four (4.1) incidents per year during the 2000s (6.4 victims murdered, 4.0 wounded per incident), and four (4.5) incidents per year from 2010 through 2013 (7.4 victims murdered, 6.3 wounded per incident).
These decade-long averages suggest that the prevalence, if not the deadliness, of "mass public shootings" increased in the 1970s and 1980s, and continued to increase, but not as steeply, during the 1990s, 2000s, and first four years of the 2010s.
Mass shootings are arguably one of the worst manifestations of gun violence. As discussed in this report, statute, media outlets, gun control and rights advocates, law enforcement agencies, and researchers often adopt different definitions of "mass killing," "mass murder," and "mass shooting," contributing to a welter of claims and counter-claims about the prevalence and deadliness of mass shootings. With improved data, policymakers would arguably have additional vantage points from which to assess the legislative proposals that are inevitably made in the wake of these tragedies.
Toward these ends, Congress could consider directing one or several federal agencies, including but not limited to the FBI and BJS, to improve collection of data on multiple-victim homicides. Congress could also direct federal agencies, possibly the Bureau of Alcohol, Tobacco, Firearms and Explosives, to report annually on firearms-related mass murders, including data on (1) offender acquisition of firearms, (2) types of firearms used, (3) amounts and types of ammunition carried and shots fired, (4) killed and wounded counts, (5) offender histories of mental illness and domestic violence, and (6) victim-offender relationships.