Procedures vary in different areas, and I know nothing of the whole story on this one- but an LEO that is in a "shots fired" situation does NOT need a warrant prior to taking action. Falls into the category of a felony committed in his presence.
Years back, had a drunk that punched his girlfriend in the face when I was about 15 ft away from him- and placed him under arrest on the spot. Yes, there was a warrant issued- after he got to the lockup. He complained to the judge that I did not have a warrant when I arrested him- judge explained that his case fell under the "Stupid in Public" rule.
That should be a real offense. Imagine what could be done!