The United States Department of Homeland Security defines active shooter as, "an individual actively engaged in killing or attempting to kill people in a confined and populated area; in most cases, active shooters use firearms and there is no pattern or method to their selection of victims. Most incidents occur at locations in which the killers find little impediment in pressing their attack. Locations are generally described as soft targets, that is, they carry limited security measures to protect members of the public. In most instances, shooters commit suicide, are shot by police, or surrender when confrontation with responding law enforcement becomes unavoidable." According to New York City Police Department (NYPD) statistics, 46 percent of active shooter incidents are ended by the application of force by police or security, 40 percent end in the shooter's suicide, and in only 14 percent of the time the shooter surrenders.
An active shooter incident is a slightly narrower definition of a mass shooting in which an individual begins shooting at people "in a confined and populated area," according to the FBI and in which law enforcement or a citizen response can intervene.
There is a considerable amount of overlap between mass shootings and active shooter incidents. The number of people killed or injured generally defines mass shootings. Active shooter incidents, on the other hand, are defined only by attempts to kill or wound people.
The FBI has recently released data that shows active shooter incidents are on the rise and have actually reported, "We now live in the age of public shootings." As shown in the graph in Figure 1, the annual number of "active shooter" incidents has continued to rise since 2000, when there was only one incident listed in the FBI database. However, active shooter incidents date back as far back as August 1, 1966, when ex-Marine sniper Charles Whitman killed his wife and mother, then climbed a 27-story tower at the University of Texas and killed 16 more people before police shot him to death.