A gunman carrying a rifle, handgun and possible suicide vest opened fire on party-goers at a "gay" nightclub in Orlando, Florida, killing at least 50 people, wounding dozens and taking several others hostage before being shot dead by a SWAT team. It is the deadliest mass shooting in the history of the U.S.

The shooting unfolded at Pulse, described on its website as the "hottest gay bar in Orlando." Witnesses said they heard at least 40 shots fired inside the club. At 5 a.m. authorities used an explosive device to distract the gunman, then authorities plowed through the club with a vehicle.

Police originally said around 20 people were dead, but Orlando Police Chief John Mina said once authorities were able to get into the club, many more victims were discovered. Mina called the attack "one of the worst tragedies we've seen."

In addition to the slayings, at least 53 other victims were taken to area hospitals for medical treatment, police said. There were an estimated 320 people in the club when it opened Saturday night.

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Orlando Mayor Buddy Dyer has declared a state of emergency for the city and the state.

"We are a united community and what we saw last night does not reflect what we feel in our hearts and souls," said Orange County Mayor Teresa Jacobs.

Law-enforcement sources told CBS News the gunman has been identified as Omar Mateen, a U.S. citizen from Port St. Lucie, a city approximately 120 miles southeast of Orlando. Mateen was born to Afghan parents in 1986. The gunman was believed to have "leanings toward radical Islamic terrorism."

Mateen had active security officer and firearm licenses, according to Florida records. Marriage records show he was married in Port St. Lucie in 2009.

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Mir Seddique, Mateen's father told NBC News, "This has nothing to do with religion." Seddique said his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting.

"We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," Seddique said.

CNN anchor Jake Tapper said during a live broadcast that the network would no longer identify Mateen during news coverage of the shooting aftermath: "We are now learning more about the suspect, learning more about him, let us use the information that I'm about to share with you sparingly, his name was Omar Mateen, that is a name I will not mention again for the rest of the show. And that is his photograph, we know he was training as a private security guard … let us take that graphic down."

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Mina said an officer was working extra duty at the club and heard reports of shots fired. He engaged the shooter, who went back inside the club and created a hostage situation, Mina said.

Mina said a decision was made to rescue the hostages, and the shooter was killed in a gunfight with nine officers, one of whom suffered a minor eye injury after a bullet struck his Kevlar helmet.

"His Kevlar helmet saved his life," said Mina.

The White House released the following statement about the shooting: "The President was briefed this morning by Lisa Monaco, Assistant to the President for Homeland Security and Counterterrorism, on the tragic shooting in Orlando, Florida. Our thoughts and prayers are with the families and loved ones of the victims. The President asked to receive regular updates as the FBI, and other federal officials, work with the Orlando Police to gather more information, and directed that the federal government provide any assistance necessary to pursue the investigation and support the community."

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Daily Mail UK notes "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."

NBC News confirmed from law-enforcement officials that Mateen had called 9-1-1 moments before the shooting to pledge allegiance to the leader of ISIS.

Mateen also referenced the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, at the scene, sources said.

The massacre took place less than four miles from where "The Voice" singer Christina Grimmie, 22, was shot dead on Friday. Mina said the massacre is not related to Grimmie's death.

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The club’s Facebook page warned: "Everybody get out of Pulse and keep running."

Kenneth Melendez was at the club with his friends when the shooting happened. He said four of his friends were shot and were treated at the Orlando Regional Medical Center.

"It's just something that you see in the movies but never think would happen to you," he said. "At first, when I heard the shots I thought it was part of the music, but then we realized it was really happening. I started running and saw someone bleeding from the arm and I was like wow this is really happening and I kept running."

He said he went to Pulse often because it was a "safe environment where you could express yourself. Not anymore."

June is the beginning of the Muslim holy month of Ramadan. In an Arabic message released online, purportedly from ISIS spokesman Abu Mohammed al-Adnani, further attacks were urged against the West during Ramadan.

The State Department-led Overseas Security Advisory Council report reads that "martyrdom during the month may hold a special allure to some," and has asked U.S. organizations to "remain aware of the persistent threat of (ISIS) attacks, both inspired and directed," according to published reports.

"The terrorism risk at the moment is generally high," the report cautioned.

ISIS reportedly cheered the Orlando massacre and promised to "slay everyone of you O disbeliever."

"Tonight our community witnessed or experienced a very horrific crime," Dyer told media near the scene Sunday morning. "Many lives were lost, and many more individuals were impacted by witnessing the crime."

Three days ago, ISIS Muslim terrorists released a statement on their plan to attack Florida, and even put out a list with the names of people in Florida that they plan on killing.

Authorities said there is no indication of future attacks in Florida or across the nation.

Article will be updated as events unfold.