For the purposes of tracking crime data, the FBI defines a "mass shooting" as any incident in which at least four people are murdered with a gun. This survey adopts this definition of a mass shooting, making it a representative sample of shootings in which at least four people were murdered with a gun. The survey findings indicate that mass shootings compose a small share of the total number of U.S. firearm homicides, constituting less than one percent of gun murders recorded by the FBI in 2010. The survey also found that assault weapons or high-capacity magazines were used in at least 13 (23 percent) of the incidents. These incidents resulted in an average of 14.8 people shot. A total of 135 percent more people were shot in mass killings than in other gun incidents, and 57 percent more deaths resulted from mass shootings than in other gun-related incidents. Survey findings also indicate that domestic or family violence was a factor closely connected to 57 percent of the cases, in that the shooter killed a current or former spouse or intimate partner or other family member. Eight of the shooters had a prior domestic violence charge. There was no evidence that any of the shooters were prohibited from possessing guns under Federal law due to having been adjudicated as mentally ill or involuntarily committed for treatment. The survey also addressed the role of prohibited possessors of firearms, whether the shooting occurred in a gun-free zone, whether the shooter committed suicide during the incident, whether the shooting occurred in a school, whether law enforcement or military officers were targeted, and whether the shootings occurred at a current or former workplace of the shooter.