When someone in plain clothes comes busting through your door and immediately opens fire you really don't have time to sort those niceties out. One of the most common tactics home invasion robbers/killers use is to yell "police" when they come busting through your door. That's why proper procedure is to have a tactical team serve the warrant - no question you're dealing with police when they come in. At a minimum they should have had the uniformed offices come through the door first. This one was on the cops.
The police didn't knock first? I assume that they did. I also assume they announced that they were police officers before kicking the door in.