You might want to work on your comprehension skills. The shooting had already started BEFORE they stormed the building.
The HOSTAGE NEGOTIATOR tried to ring him on his cell phone, the number which the got from the radio station he called saying he was going to start shooting in 10 seconds if the police didn't leave. At that exact moment that the negotiator tried to call him, the guy started killing, execution style, the little girls. The police then stormed the building, but he had barricaded the doors. By the time they were able to get in through the windows, he had killed himself and a bunch of children.
Imagine,10 seconds and then they heard the shooting.