...Police Chief John Mina has also admitted that some of the victims may have been hit by officers' gun fire.
However he insisted it is a part of the investigation into the horrific attack.
He said: 'I will say that is all part of the investigation. But I will say when our SWAT officers, about eight or nine officers, opened fire, their backdrop was a concrete wall. And they were being fired upon, so that is all part of the investigation.'
The decisions made by Orlando police made them targets for scrutiny among experts in police tactics.
They said the lessons learned from other mass shootings show that officers must get inside swiftly — even at great risk — to stop the threat and save lives.
'We live in a different world. And action beats inaction 100 per cent of the time,' said Chris Grollnek, an expert on active-shooter tactics and a retired police officer and SWAT team member.
Authorities in Orlando say the situation changed from an active-shooter scenario to a hostage situation once gunman Mateen made it into one of the bathrooms where club-goers were hiding.
He first had a shootout with the off-duty officer at the club's entrance.
Then two other officers arrived and the firing continued.
Experts say there's a big difference between responding to a lone gunman and a shooter who has hostages.
In active-shooter situations, police are now trained to respond immediately, even if only one or two officers are available to confront the suspect.
In a hostage crisis, law enforcement generally tries to negotiate.
Once in the restroom, Mateen called 911 and made statements pledging allegiance to the Islamic State, Orlando Police Chief John Mina said Monday.
That's when the shooting stopped and hostage negotiators began talking with him, the chief said.
'We had a team of crisis negotiators that talked to the suspect, trying to get as much information as possible, what we could do to help resolve the situation... He wasn't asking a whole lot, and we were doing most of the asking,' Mina said.
But Mateen soon began talking about explosives and bombs, leading Mina to decide about 5am to detonate an explosive on an exterior wall to prevent potentially greater loss of life.
The explosives did not penetrate the wall completely, so an armored vehicle was used to punch a two-foot-by-three-foot hole in the wall about two feet from the ground.
'We knew there would be an imminent loss of life,' Mina said.
Hostages started running out, as did Mateen, who was killed in a shootout with SWAT team members.
It turned out there were no explosives.
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