A man armed with an assault rifle and undertake loyalty to Islamic State killed 50 people during a gay pride celebration at a gay nightclub in Orlando, Florida, early on Sunday in the deadliest mass shooting in U.S. history. 

Police said the gunman who was heavily armed with guns and other assault weapons had a fire exchange with the security before entering inside the Pulse night club. The hostage situation quickly developed before a squad of officers entered the club and shot the gunman dead. One police officer has also been injured in the crossfire.

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Authorities have identified the shooter as 29-year-old Omar Mateen of Fort Pierce, Florida. In a 911 call from the club, Mateen professed allegiance to the leader of the Islamic State, Abu Bakr al-Baghdadi. He was a bodybuilder and a security guard, a religious man who attended the local mosque and wanted to become a police officer. Omar Mateen was a US citizen born in New York. His ex-wife said his family was from Afghanistan.

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Just a few days earlier, Orlando’s vibrant LGBT community had marked the annual Gay Days celebration, one of the biggest events anywhere in the United States dedicated to gay pride. The partying was to go on at Pulse, a popular dance club and bar known for its drag shows.

Several person had posted on social media that a gunman was holed up inside and holding hostages. One man who said he was inside the club posted that the shooting broke out around 2 a.m. and that he heard about 40 shots being fired. Javer Antonetti, 53, told the Orlando Sentinel newspaper that he was near the back of the dance club when he heard the gunfire. “There were so many (shots), at least 40,” he said. “I saw two guys and it was constant, like ‘pow, pow, pow,’.”

Police said on Twitter they had carried out a “controlled explosion” at the club hours after the shooting broke out, but did not say why that was done. They described the scene as a “fluid situation”.

A video posted online showed a large number of police and emergency vehicles outside the nightclub. Bomb sniffing dogs were also on the scene, CNN reported.

The FBI said the shooting was “an act of terrorism.” Even though they have confirmed that the situation is under control, a state of emergency has been declared in Florida.