The National Tactical Officers Association defines active shooter as one or more subjects who participate in a random or systematic shooting spree, demonstrating their intent to continuously harm others. An active shooter’s overriding objective appears to be that of mass murder, rather than criminal conduct such as robbery, kidnapping, etc. The definition of an active shooter can include any assault with a deadly weapon causing a mass homicide.
The history of active shooter incidents is very dynamic and no one set of procedures or guidelines can cover every single aspect. An active shooter incident may quickly turn into a hostage/barricade situation and conversely a hostage/barricade situation may quickly turn into an active shooter incident. When an individual has arrived on a campus with the means and motivation to commit a shooting spree the percentages indicate that they will continue their action until they are confronted by a greater force.
Most active shooter incidents have been committed by a lone assailant; some have been committed by multiple assailants. Many of the active shooter incidents have been committed in extreme close quarters, while a few have been committed at longer distances by a sniper.
If you think you heard gunshots then treat it as gunshots. If you think you saw a person with a weapon then act upon it. Trust your intuition. Remaining calm and using common sense will increase your ability to survive a critical incident such as this.