-- Deadliest mass shooting in U.S. history

-- Gunman killed 49 people at a gay nightclub in Orlando, Florida. At least 53 others taken to nearby hospitals.

-- Propaganda outfit tied to ISIS claimed responsibility for the attack

-- Club posted on its Facebook page, "Everyone get out and keep running"

A gunman opened fire early Sunday at a gay nightclub in Orlando, Florida, killing 49 people and wounding at least 53 others in the deadliest mass shooting in U.S. history. 

A propaganda outfit tied to ISIS claimed responsibility for the massacre on Sunday afternoon following reports that the shooter had called 911 just before the attack to pledge allegiance to the terrorist organization.

Authorities said that the shooter, Omar Mateen, 29, entered the Pulse nightclub around 2 a.m. EST, briefly exchanging gunshots with an off-duty police officer before entering the club.

At 5 a.m. local time, police decided to attempt to rescue the hostages. Police confirmed about an hour later that the gunman had been killed. That update followed a controlled explosion by authorities.

Mateen had previously been investigated by the FBI and interviewed twice, but the findings were deemed inconclusive. An ATF representative said Mateen acquired the weapons used in the attack legally.

"It's appears he was organized and well-prepared," Orlando police chief John Mina said at a press conference.

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While ISIS has claimed responsibility, those that closely watch the terrorist organization and its propaganda outfits noted differences in how the news was broken. There is no indication, they added, that ISIS directed the attack or knew about it in advance.

Mateen used an "AR-15-type assault rifle," Mina said. AR-15 rifles have been used in many of the most recent and high-profile U.S. mass shootings including Aurora, Colorado, Sandy Hook, Connecticut, and San Bernardino, California.

Earlier in the day, Mateen's father insisted that the shooting had "nothing to do with religion," telling NBC News that his son had recently been upset when he saw two gay men kissing recently in Miami. Mateen's father, however, clarified that he did not know why his son had perpetrated the attack.

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"We weren't aware of any action he is taking. We are in shock like the whole country," he told NBC News.

Governor Rick Scott declared a state of emergency for Orange County, Florida. Miami Beach police said they would be increasing patrols around locations that are known to be frequented by the LGBT community.

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According to Mina, the gunman was carrying an assault rifle and a handgun, along with an unidentified device. The police also checked vehicles in the vicinity for explosives. 

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The shooting comes amid LGBT Pride Month, which features major events in cities across the U.S.

As the mass shooting was unfolding, Pulse Nightclub posted a status update on Facebook around 2 a.m. local time Sunday telling people to run from the club. "Everyone get out of pulse and keep running," the post declared. 

Patrons replied on the Facebook update with various confirmations of the violence.

Other patrons used Twitter call for help and described a disturbing scene, with many hiding in different areas of the club.

"Never seen so many dead bodies on the floor, God is good that my friends and I didnt get shot," witness Juan Rivera wrote in a now-deleted tweet.

"PLESSE TELL THE COPS WERE WT PULSE ORLANDO HIDING IN THE DRESSING ROOM THERE WAS A SHOOTING," Twitter user GoEmili0 wrote.

"FOUR OF US STILL HIDING. LIGHTS ARE OFF IN CLUB. COPS ARE HERE BUT HAVNT GOT US YET."

Ricardo Almodovar, who claimed he was present at the club at the time of the shooting, wrote on Facebook that a shooter opened fire inside the busy venue. 

"People on the dance floor and bar got down on the floor and some of us who were near the bar and back exit managed to go out through the outdoor area and just ran. I am safely home and hoping everyone gets home safely as well," he wrote. 

After the attack, footage emerged from the scene, showing a large police presence as ambulances arrived at the nightclub.

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#BREAKING VIDEO: Mass shooting at Pulse night club in Orlando. 20+ victims being treated pic.twitter.com/g6bLM6JP17

— Joel Franco (@OfficialJoelF) June 12, 2016

President Barack Obama and Vice President Joe Biden were reportedly briefed on the shooting and are receiving regular updates, according to a statement from the White House.

Facebook activated its safety check feature in the wake of the shootings, deploying the feature for the first time in the United States.

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Editor's note: The death toll has been updated to 49, to exclude the shooter, per the FBI on Monday.

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