The first of 50, in my opinion. I just had active shooter training for my job (not in LE at all, just a regular state worker). Active shooter training is becoming a huge deal, not just for LE but for the general public.
The LE officers training us told us of a shift in tactics since Columbine. The old tactics involved surrounding and securing the building in question, waiting for the arrival of special ops (such as SWAT), and taking the building en masse. However, in the last 4 years their tactics have changed. There is no more waiting. The first officers to arrive at the scene are to immediately enter the building and stop the shooter(s). That means every single officer needs to be fully trained on dealing with an active shooter in a building.
Our instructors told us how France is now learning why this change in tactic is necessary. During the attack in the concert theater in Paris a couple months ago, the police used the surround and secure the building tactic, which allowed the shooters a full 40 minutes inside by themselves to just keep shooting. If the first responders had gone inside when they got there, it could have possibly been over in 10 minutes or less, likely saving lives.