The shooter, Omar S. Mateen (Orlando Police)
A police officer working as a security guard inside the Pulse nightclub in downtown Orlando exchanged fire with the suspect at about 2am on Sunday, police officials said.
The gunman then entered the club and a hostage situation quickly developed. Three hours later a squad of officers stormed the club and shot dead the gunman.
The nightclub posted on Facebook at the time: "Everyone get out of Pulse and keep running."
Orlando police called the incident a mass casualty situation, telling people to "stay away from the area". A crisis negotiator was sent to the scene.
"Do we consider this an act of terrorism? Absolutely, we are investigating this from all parties' perspective as an act of terrorism," said Danny Banks, special agent in charge of the Florida Department of Law Enforcement.
"Whether that is domestic terrorist activity or an international one, that is something we will certainly get to the bottom of."
When asked if the FBI suspected the gunman might have had inclinations toward militant Islam, including a possible sympathy for Islamic State, Ronald Hopper, an assistant FBI agent in charge, told reporters: "We do have suggestions that the individual may have leanings toward that particular ideology. But right now we can't say definitively."
The FBI said it was still trying to pin down whether the mass shooting was a hate crime against gay people or a terrorist act.
President Barack Obama ordered the federal government to provide any assistance needed to Florida police investigating the shooting, the White House said in a statement.
The attacker was carrying an assault rifle and a handgun, Orange County Sheriff Jerry Demings said. He was also carrying an unidentified "device", Orlando Police Chief John Mina said earlier.
Javer Antonetti, 53, told the Orlando Sentinel newspaper that he was near the back of the dance club when he heard gunfire. "There were so many (shots), at least 40," he said. "I saw two guys and it was constant, like 'pow, pow, pow'."
The choice of target was especially heart-wrenching for members of the US lesbian, gay, bisexual and transgender community, LGBT advocacy group Equality Florida said in a statement.
"Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety," the group said.
"Today we're dealing with something that we never imagined and is unimaginable," Orlando Mayor Buddy Dyer said.
It was the second deadly shooting at an Orlando night spot in as many nights.
Late on Friday, a man thought to be a deranged fan fatally shot Christina Grimmie, a rising singing star and a former contestant on "The Voice", as she was signing autographs after a concert in the central Florida city.
In an earlier unrelated event in Los Angeles, a man was arrested with an arsenal of weapons close to a gay pride parade.
"It's not only tragic, it's actually quite terrifying for people," Prime Minister John Key told Paul Henry on Monday morning.
"I remember talking to [daughter] Stephie who was in Paris during the shootings there, the terror attacks late last year. A couple of her friends were down at Bataclan, where there were a lot of people shot.
"People get terrified of going out because they start thinking if it's possible here, why not anywhere else? It's a terrible day for the United States. It's a tragic day for the gay community, but it's also just an awful day for Americans in general who have to live with it."
Newshub. / Reuters