ORLANDO — After an initial burst of fire between Omar Mateen and a security guard at the Pulse nightclub, a group of five or six police officers arrived on the scene within minutes, broke through a large glass window and entered the club as the killing of 49 people was underway inside, according to a Belle Isle, Fla., police officer who was among the first responders. Officer Brandon Cornwell, 25, said the ad-hoc team spent the first seconds in the dimly lit club “trying to locate exactly where the shooter was — we kept hearing people scream and shots fired.”...