It's true. In both advanced individual training (the job specific one that follows the famous "basic training") and before deploying to Afghanistan, I underwent a good amount of tactical and combat training. In both situations we were not only taught proper protocols for "escalation of force" and "rules of engagement" but also threatened with doom if we didn't follow them.

It varies on many factors, but generally before pulling the trigger you have to visually identify a threat, yell a verbal warning, show your weapon as a warning, fire a warning shot, aim at the threat and repeat the verbal warning, then fire if necessary.

Obviously this doesn't apply to full-combat situations, but for instances of the type that our domestic police force encounter, it most certainly would. It's so ingrained that I always wonder how we have so many occurrences of police shootings.

Sad situation, especially because (imho) the vast majority of police are amazing, selfless, community minded heroes, whose profession is besmirched by these brutal thugs who share their uniform.