There are two ASEs in particular that were watershed moments for fire and police departments. The first was the 1999 Columbine High School ASE in Littleton, CO. At the time, law enforcement was trained to secure the perimeter of the building and wait for tactical teams to arrive to neutralize the shooter. But as a result of this event, law enforcement personnel realized that they could no longer wait outside for tactical teams to arrive, because more lives may be taken during the wait time. The average ASE lasts 12 minutes, with 37 percent lasting less than 5 minutes. Forty percent of the incidents end with the shooter committing suicide.
The concept of command and control of an active shooter event requires that fire and police operations become integrated as a combined unit in order to properly mitigate the incident in a coordinated fashion.
Source: Firehouse Magazine