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Source: Ready Houston

Run: If it is safe to do so, get away as quickly as possible. Leave belongings behind and try to help others escape if possible. Keep your hands visible and follow the instructions of law enforcement officers when help arrives.
Hide: If you cannot safely evacuate, try to hide in a secure location that is out of the shooter's view. Lock or barricade the doors and turn off the lights. Stay quiet and avoid making any noise that could alert the shooter to your presence.
Fight: If confronted by the shooter, use any available means to defend yourself and others. This may include using chairs, fire extinguishers, or other objects to distract or disarm the shooter.Call 911 as soon as it is safe to do so and provide any relevant information to law enforcement.