Today seems like a good day to repost this one. I’ll note that the only historically proven way to stop an active mass murder in-progress is for an armed solo responder, be they “civilian” or cop, to start shooting back:
Solo response to the active-shooter
Although I wrote this bit a few years ago, nothing in the mean time has changed my opinion on the viability, and critical need for, armed good guys to be ready to act alone to stop murderers.
Also, when the smoke clears, having some trauma first aid training is a really good thing to have had in the data bank.