When an active shooter situation is in progress, what is usually required to stop the shooter and mitigate harm to victims?

An active shooter is an individual actively engaged in killing or attempting to kill people in a confined space or other populated area. In most cases, active shooters use firearms and there may be no pattern or method to their selection of victims. Active shooter situations are unpredictable and evolve quickly. Typically, the immediate deployment of law enforcement is required to stop the shooting and mitigate harm to victims. Active shooters usually will continue to move throughout a building or an area until stopped by law enforcement, suicide, or other intervention.

What was the length to begin with?