Autrey, A., Hick, J., Bramer, K., et al. (2014). 3 Echo: Concept of Operations for Early Care and Evacuation of Victims of Mass Violence. (Abstract only.) Prehospital Disaster Medicine. 29(4):421-8.
The authors describe a three-phase approach first responders can use when responding to a blast or active shooter event: Enter, Evaluate, and Evacuate, or 3 Echo.