You can download the Department of Homeland Security’s (DHS) publication on how to respond to an active shooter here. Additional information and resources are available on the Cybersecurity & Infrastructure Security Agency website.
Law enforcement agencies across the country promote three basic guidelines about what to do if an active shooting happens near you: Run, Hide, and Fight.
Run (Escape) away as soon as you can determine where the shooting is taking place. Be mindful that there may be more than one shooter and also that the location of the shooter can change quickly. Avoid running to your car to leave. Cars jammed up trying to escape are a stationary target. It is better if you keep moving until you are safely off property.
Hide (Shelter in Place/Barricade) when it is not practical to run or escape because you may be targeted by the active shooter. If possible, hide by barricading yourself in a lockable area like a room. Some rooms may not have a lock on the door so move heavy furniture or anything else which can help to fortify the door from the attacker opening it. If there is a window in the door to the room, cover it so a shooter cannot see if the room is occupied. An active shooter will suffer tunnel vision like anyone else under stress and will not spend a lot of time on barricaded doors because of the goal to kill as many as possible then themselves.
Fight or attacking the active shooter is an option if you are cornered or trapped. It is recommended to use something that will puncture or can be used as an impact weapon against the attacker. Puncturing objects are all around whether at home, work or a mall. Think about using ink pens, scissors and letter openers all which can be used against the attacker. Nontraditional impact weapons such as coffee mugs, staplers, fire extinguishers, a chair or any heavy object can be very effective in stopping the shooter, Mass attacking the active shooter on entry into a room is the fastest and most efficient way to stop the attack because the superior numbers counterattacking. If the attacker bursts into the room without warning and there is no exit for escape, then Fight is the only option. Prepare your mind to not hesitate grabbing anything you can throw at the attacker’s face causing them to flinch even if just briefly. Grab the weapon while shouting to others to get the legs! This will encourage others to join in with you. If there is time to barricade the room, then take charge by directing everyone to get anything that can be used to attack the attacker. Tell everyone to get on either side of the door and as soon as the attacker enters, direct some to grab the end of the gun, others the legs. Once the attacker is stopped, look for anything to bind them behind their back until police arrive.
For further reading, search Crisis Rehearsal and Tactical Breathing by Dave Grossman.