Maybe this will clue you in
Although it does claim that a paramedic came outside and told the Firechief that no surivors left inside. Firechief is not the one that made that determination.
Your own link answers the question.
The first on the scene were nine Newtown officers, divided into three teams of three, including the police chief. They were the first, after the shooter, to force their way into the school, via the front lobby and the rear door.