Everyone should read this entire thing. It's long, but worth every word.
Cliff's notes: resistance is your only hope and saves lives.
Active Shooter Research - Hard Tactics BlogExcerpt from the middle of the blog:
The Amish School Massacre, Bart Pa, October 2006. This is one of the most botched active shooting incidents I know of. At the point in the incident, when police, SWAT and EMS were on the scene, and the perpetrator began firing, two state troopers requested permission to enter the building. Permission was denied. Police did not enter the building until the shooting stopped! Police report there were 17/18 shots fired. However, the coroner reported at least 24 bullets in one victim alone. No case points to the need of immediate assault upon active shooting more than this case. Police and SWAT were completely unprepared to make entry or even to make a decision. Ten State Troopers received the State Police Medal of Honor…The children received nice funerals…and the assailant received forgiveness from the Amish community.
