The gunman opened fire on the dance floor inside Pulse nightclub, Orlando, at around 2 am leaving about 50 people dead and 53 others wounded, officials confirmed late Sunday morning.
The SWAT team made the decision to rescue the hostages at approximately 5 am, according to officials.
The nine hero officers used a "controlled explosion" to distract the shooter before fatally shooting him and were able to rescue about 30 hostages who were hiding in the bathroom of the club.
The gunman had an assault rifle, handgun and "some kind of device on him", say officials.
During the tense shoot-out a bullet struck an officer's Kevlar helmet, which Police Chief John Mina believes saved the officer's life.
The moment police officers took the gunman was captured on video taken by bystander Nic Hornstein from a rooftop across the road from the club.
The mass-killer was shot dead just before 6am, according to police, who said the incident was a "mass casualty situation."
Mayor Buddy Dyer said in a press conference: "Many were saved by the heroic efforts of the men and women of the OPD, the Orange County Sheriffs, Seminal County Sheriff's office."
There were about 320 people inside the club at the time of the shootings, officials said.
Jon Alamo said he was inside the club, which was hosting a Latin-themed night, when a man holding a weapon entered the room he was in.
"I heard 20, 40, 50 shots," Alamo said. "The music stopped."
Club-goer Rob Rick said it happened around, 2 a.m., just before closing time. "Everybody was drinking their last sip," he said.
An FBI spokesman said the mass shooting is being investigated as an act of terrorism. He explained authorities are looking into whether this was an act of domestic or international terror, and if the shooter was a lone wolf.
Police said the gunman, who was shot dead, was believed to be in his 20s was not a local man, while the FBI believe he may have "leanings to radical Islamic terrorism".
The FBI was among the agencies at the crime scene along with the Orlando Fire Department's bomb squad and hazardous material team.
The killings took place less than four miles from where The Voice singer and YouTube star Christina Grimmie, 22, was shot dead at The Plaza Live in Orlando on Friday.
