according to the BBCThis may have been mentioned before, but I had another thought. This officer didn't go in for something like 4 minutes, and I would have thought that he might have wanted to be inside even if for no other reason than to try to determine where the shooter was and how he was armed. Even if he couldn't respond himself (after all, students were probably running everywhere in the hallways), he could have at least scoped out the situation so that he could inform other responding officers as to the shooter's location and "possible" armament.
I'm pretty sure that the responding officers would have like to have had that information.
"It has been reported that the person watching the surveillance system was relaying information 20 minutes old to police, so officers believed the gunman was in a certain area when he was not."
http://www.bbc.com/news/world-us-canada-43164634