From the NY Times:
The police, by now swarming outside, had established communication with Mr. Mateen, but it was unclear what they discussed. Heavily armed members of the SWAT team took positions around the building while other emergency medical workers rushed the wounded to a hospital, which was just a block away.
Finally, around 5 a.m., the authorities took action.
They breached the wall of the isolated room with an armored personnel carrier known as a Bearcat.
“That is when the shooter opened the door and started shooting outside,” Mr. Dyer said.
One officer was shot in the head, but was not seriously wounded because the bullet struck his Kevlar helmet.
From the Washington Post:
Authorities declined to offer details. But a senior U.S. law enforcement official said officers delayed their assault on the gunman because the active-shooter scene turned into a hostage negotiation once the gunman called 911. For three hours, the gunman was on the phone with police and no shots were fired.
...“That is when you do wait,” said the official, who spoke on the condition of anonymity because he was not an authorized spokesman. “It was appropriate.”
Police ultimately chose to end the standoff because of concerns about the health and safety of dozens of people who were injured and trapped inside. Officials at nearby Orlando Regional Medical Center said at least nine people died at the hospital or were dead when they arrived. It was unclear whether anyone was injured during the gunman’s final shootout with officers, although authorities said that one Orlando police officer was shot but his Kevlar helmet saved him.