Having just been through an Active Shooter drill with Nashua PD at Pheasant Lane Mall this past Sunday evening, I can say that’s how they trained. In the drill, the 1st responding unit entered quickly but with less urgency, with pistols only, as it wasn’t a shooting event yet. They didn’t find us - Mall Security took them the perp’s 1st floor route via comms to the video cam room rather than to the current scene on the 2nd floor. The next unit responded with rifles and moved hard and fast. Of course, there were PD trainers/observers and the responding teams were being judged & graded, so they had to go hard and fast.
All they can do is plan to fight like they train. Run toward the gunfire is a great response plan but people gotta be getting shot to guide them in. It’s that body count in the first minutes that is hard to control without armed and determined security on site.