Orlando police are facing questions over why it took three hours for a SWAT team to storm the nightclub where ISIS fanatic Omar Mateen slaughtered 49 people.

As the largest mass shooting in modern U.S. history began to unfold, an off-duty police officer working at a gay nightclub exchanged gunfire with the suspect.

But authorities chose to hold off on letting the tactical units enter Pulse nightclub, where more than 100 people were shot.

Police Chief John Mina has also admitted that some of the victims may have been hit by officers' gun fire.

Orlando Police Chief John Mina has admitted that some of the victims inside Pulse nightclub could have been shot by law enforcement as his department faces questions over why they told SWAT teams to wait for three hours after gunman Omar Mateen had opened fire

Two bodies of victims arrive at the Orlando Medical Examiner's Office. They were among the 49 people killed in the horrific attack in the early hours of Sunday morning

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.

VICTIMS: 48 IDENTIFIED SO FAR On Sunday, Edward Sotomayor Jr., 34; Stanley Almodovar III, 23; Luis Omar Ocasio-Capo, 20; Juan Ramon Guerrero, 22; Eric Ivan Ortiz-Rivera, 36; Peter O. Gonzalez-Cruz, 22; Luis S. Vielma, 22 and Kimberly Morris, 37, were confirmed dead. In the early hours of Monday morning, Eddie Jamoldroy Justice, 30; Darryl Roman Burt II, 29, Deonka Deidra Drayton, 32; Alejandro Barrios Martinez, 21; Anthony Luis Laureanodisla, 25; Jean Carlos Mendez Perez, 35; and Franky Jimmy Dejesus Velazquez, 50, were added to the list. Later on Monday morning the authorities announced more names of the 50 who died: Amanda Alvear, 25; Martin Benitez Torres, 33; Luis Daniel Wilson-Leon, 37; Mercedez Marisol Flores, 26; Xavier Emmanuel Serrano Rosado, 35; Gilberto Ramon Silva Menendez, 25; Simon Fernandez, 31; Oscar Aracena-Montero, 26; Enrique L. Rios, Jr, 25; Miguel Angel Honorato, 30; Javier Jorge-Reyes, 40; Joel Rayon Paniagua, 32; Jason Benjamin Josaphat, 19; Cory James Connell, 21; Juan P. Rivera Velazquez, 37; Luis Daniel Conde, 39; Shane Evan Tomlinson, 33; Juan Chevez-Martinez, 25; Jerald Arthur Wright, 31; Leroy Valentin Fernandez, 25; and Tevin Eugene Crosby, 25. In the afternoon, authorities identified another nine victims: Jonathan Antonio Camuy Vega, 24; Jean C. Nives Rodriguez, 27; Rodolfo Ayala-Ayala, 33; Brenda Lee Marquez McCool, 49; Yilmary Rodriguez Sulivan, 24; Christopher Andrew Leinonen, 32; Angel L. Candelario-Padro, 28; Frank Hernandez, 27; Paul Terrell Henry, 41; Antonio Davon Brown, 29; Christopher Joseph Sanfeliz, 24; and Akyra Monet Murray, 18. Advertisement

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.

Police tactics changed after the 1999 massacre at Columbine High School, where the first officers to arrive exchanged fire with the gunmen but then stopped and waited for the SWAT team. That took 45 minutes. By then, Eric Harris and Dylan Klebold had killed 12 students and a teacher.

Police forensic workers and a member of the FBI examine holes that were blown in the side of the nightclub during the gun battle with Mateen

A medical worker examines the body of one of the victims of the horrific nightclub shooting. Orlando Police are now investigating whether delays in their decisions gave the gunman more time

ORLANDO SHOOTINGS TIMELINE 2:02am - Sunday, June 12 - Suspect Omar Mateen tries to enter Pulse nightclub armed with assault rifle and and handgun and is spotted by a uniformed officer. The two exchange fire outside the club - but Mateen manages to enter the nightclub holding 320 people while still firing. The shooting then developed in a hostage situation. 2:09am - Pulse nightclub posts a message to their Facebook desperately telling those inside 'Everyone get out of pulse and keep running'. 5.05am - Three hours after the first shooting began, a SWAT team rushed inside to rescue hostages and confront Mateen. Members of the SWAT team detonated a controlled explosion to confuse Mateen and then nine officers exchange gunfire with terrorist - shooting him dead. SWAT officers manage to free 30 hostages who dashed to safety. 5:35am - Orlando police confirm that the shooter is dead. 8:00am - Officials with the FBI deem the incident to be 'domestic terrorism'. 9:40am - The shooter was officially identified as 29-year-old Omar Mateen of Port St. Lucie. 10:20am - At a press conference Mayor Buddy Dyer makes the announcement that 49 people have died and 53 injured in what is now the worst mass shooting in the history of the United States. 11.54am - US Senator Bill Nelson and California congressman Adam Schiff claimed Mateen had pledged his allegiance to ISIS Advertisement

At the time, the standard police practice was to set up a perimeter, wait for SWAT officers and then go inside. Authorities began to realize that the delayed response gave suspects more time to kill.

'We can't just let him have free rein and continue to shoot,' said Ben Tisa, a former FBI agent and former SWAT team member.

Experts point to other mass shootings where a delay in confronting the shooter probably gave the gunman time to maneuver and attack.

A mass shooting in 1984 at a McDonald's in San Ysidro, California, offered one of the earliest lessons, with 21 people being killed and 19 wounded before a SWAT team killed the gunman about 45 minutes later.

Incorrect or incomplete information is typical during police emergencies. And the gravity of the decisions is not lost on SWAT teams and their commanders. Civilian lives are at risk, along with those of police officers who are often outgunned by suspects.

Authorities said the face-off with gunman Omar Mateen (left and right) started as an active-shooter situation, but quickly changed into a hostage crisis when he entered a toilet where revelers were hiding

'You have split seconds,' said Thor Eells, commander of the Colorado Springs Police Department and chairman of the board of the National Tactical Officers Association.

Almost immediately after the shooting began, the nightclub posted a note on its Facebook page telling people to get out and 'keep running.'

Grollnek, a consultant who conducts active-shooter training for law enforcement, said that's another lesson from other mass shootings: civilians can't expect to stay safe by heeding the old advice to hide or shelter in place.

'The problem is we're failing to evolve by learning the lesson that hiding does not work,' he said. 'Running works. Everyone who escapes to tell their story says, "I ran away. I heard a noise on my left, and I went to my right and I got out".''

But Grollneck reserved his anger for the police commanders in Florida who didn't allow SWAT team members to enter until several hours after the shooter began the attack.

'How have we failed so poorly that we did not learn our lesson... when we see SWAT teams respond and not making entry creates victims,' he said. 'Period. End of story.'

A soldier stands guard outside the Pulse nightclub, where a gunman opened fire in the early hours of Sunday morning

Witnesses said he fired relentlessly - 20 rounds, 40, then 50 and more. Pictured above, the damaged rear wall of the nightclub is seen

Forensic tents are set up around the nightclub as authorities begin their investigation into the massacre