What I want know is WHY was a rookie (regardless of age) out there alone.
Where was his training officer?
Watch the video. From my perspective 2 cops are running across the parking lot, what appears to be the training officer drops something, delaying him a few quick seconds as the recruit now almost to the door. The recruit enters first and the shooting immediately followed with FTO entering about 2 or 3 seconds later. Just my take on it.
If this is correct, the recruit knew FTO was about 30 feet behind him when he entered the building. Policy violation. I'd bet there is more to this.