There are several major incidents that changed the way US law enforcement agencies choose equipments. One of them was 1986 Miami shootout resulting in deaths of 2 Special Agents and 2 bank robbers.
The incident was the catalyst for FBI's change to 10mm, followed by .40 S&W, which lead to nationwide conversion to .40 S&W as the firearm of choice.
Here's the video FBI produced and showed to their agents in training
