I didn't see this anywhere else, apologies if it's a dupe.
FBI Report: Active Shooter Incidents in the United States in 2016 and 2017
(Also available for download here.)
The FBI defines an active shooter as one or more individuals actively engaged in killing or attempting to kill people in a populated area. Implicit in this definition is the shooter’s use of one or more firearms. The active aspect of the definition inherently implies that both law enforcement personnel and citizens have the potential to affect the outcome of the event based upon their responses to the situation.
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...Excluded from this report are gang- and drug-related shootings and gun-related incidents that appeared not to have put other people in peril (e.g., the accidental discharge of a firearm in a bar).
Summary: There were 50 "active shooter" incidents in 2016 and 2017; of these, 20 met the FBI's definition of "mass killing".
There were 943 casualties "excluding the shooters."Twenty of the 50 incidents met the criteria cited in the federal definition of “mass killings,” that is, “three or more killings in a single incident.”
13 LEOs were killed, and 20 wounded.
13 shooters committed suicide, 11 were killed by police, and 8 were "stopped" by citizens.
Citizen Engagement and Casualties
In 10 incidents, citizens confronted the shooter. In eight of those incidents, one or more citizens safely and successfully acted to end the shooting.
■In four incidents, unarmed citizens confronted or persuaded the shooter to end the shooting. In twoincidents,17 school staff confronted and restrained the shooter. In one incident, the citizen used his car to thwart the shooter. In one incident, the citizen persuaded the shooter to surrender via telephone during a police chase; she ran up to the shooter’s car as he came to a stop and pulled him out of his seat, bringing the chase to an end.
■In four incidents, citizens possessing valid firearms permits successfully stopped the shooter. In two incidents,20 citizens exchanged gunfire with the shooter. In two incidents, the citizens held the shooter at gunpoint until law enforcement arrived.
■In one incident, a citizen possessing a valid firearms permit exchanged gunfire with the shooter, causing the shooter to flee to another scene and continue shooting.
■In one incident, a citizen possessing a valid firearms permit was wounded before he could fire at the shooter.