From the Introduction: "An analysis of 2014 and 2015 active shooter incidents has identified 20 incidents in each of the years; information provided to advance available research and assist federal, state, local, tribal, and campus law enforcement officers, other first responders, corporate leaders, and educators in their efforts to prevent, prepare for, and respond to active shooter incidents. This annual tally is an increase from 17 in the year 2013, which was the last year of a 14-year study released by the FBI. 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 firearms when engaging in criminal activity."
Federal Bureau of Investigation: https://www.fbi.gov