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NEW: Suspected Orlando shooter called 911 after the shooting began to pledge his allegiance to ISIS, law enforcement officials tell @ABC.— ABC News (@ABC) June 12, 2016
Authorities were investigating the attack on the Florida dance hall as an act of terrorism. The gunman's father recalled that his son recently got angry when he saw two men kissing in Miami and said that might be related to the assault.
At least 53 people were hospitalized, most in critical condition, officials said. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.
Mayor Buddy Dyer said all of the dead were killed with the assault rifle.
"There's blood everywhere," Dyer said.
Witnesses described a chaotic scene when the gunfire began shortly before the club known as Pulse was to close.
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"Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance," said Jackie Smith, who had two friends next to her get shot. "I just tried to get out of there."
The suspect exchanged gunfire with a police officer working at the club, which had more than 300 people inside. The gunman then went back inside and took hostages, Police Chief John Mina said.
Around 5 a.m., authorities sent in a SWAT team to rescue the hostages.
Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.
"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County Sheriff Jerry Demings said.
The previous deadliest mass shooting in the U.S. was the 2007 attack at Virginia Tech, where a student killed 32 people before killing himself.
The suspect in the Orlando attack was identified as Omar Mateen of Port St. Lucie, Florida.
Rep. Alan Grayson named the shooter, citing law enforcement officials. A federal law enforcement official with knowledge of the investigation also confirmed the name. The official was not authorized to discuss the investigation publicly and spoke on condition of anonymity.
Mateen's father, Mir Seddique, told NBC News about his son seeing the men kissing a couple of months ago.
"We are saying we are apologizing for the whole incident," Seddique said. "We are in shock like the whole country."
FBI agent Ron Hopper said there was no further threat to Orlando or the surrounding area.
When asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities had "suggestions that individual has leanings towards that."
Mateen's father said the attack had nothing to do with religion, he said.
In addition to the guns, the shooter also had some sort of "suspicious device," the police chief said.
Authorities said they had secured the suspect's vehicle, a van, outside the club.
A SWAT truck and a bomb-disposal unit were on the scene of an address associated with Mateen in a residential neighborhood of Fort Pierce, Florida, about 118 miles southeast of Orlando.
Relatives and friends, many in tears, gathered outside the hospital to learn the fate of loved ones.
Smith did not know the conditions of her friends and came out of the hospital and burst into tears.
Christine Leinonen drove to Orlando at 4 a.m. after learning of the shooting from a friend of her 32-year-old son, Christopher Leinonen, who was at Pulse and is missing.
She has not heard from her son and fears the worse.
"These are nonsensical killings of our children," she said, sobbing. "They're killing our babies!"
She said her son's friend Brandon Wolf survived by hiding in a bathroom and running out as the bullets flew.
A woman who was outside the club early Sunday was trying to contact her 30-year-old son, Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: "He's coming."
"The next text said: 'He has us, and he's in here with us,'" Mina Justice said. "That was the last conversation."
Pulse posted on its own Facebook page around 2 a.m.: "Everyone get out of Pulse and keep running." Just before 6 a.m., the club posted an update: "As soon as we have any information, we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love."
President Barack Obama was briefed on the attack and asked for regular updates on the investigation, the White House said.
Jon Alamo said he was at the back of one of the club's rooms when a man holding a weapon came into the front of the room.
"I heard 20, 40, 50 shots," Alamo said. "The music stopped."
Club-goer Rob Rick said the shooting started just as "everybody was drinking their last sip."
He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area where only workers are allowed. People inside were then able to escape through the back of the club.
Christopher Hansen said he was in the VIP lounge when he heard gunshots. He continued to hear shooting even after he emerged and saw the wounded being tended across the street.
"I was thinking, 'Are you kidding me?' So I just dropped down. I just said, 'Please, please, please, I want to make it out,'" he said. "And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot."
The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who had been a YouTube sensation and former contestant on "The Voice." She was killed after a concert in Orlando by a 27-year-old man who later killed himself.
Associated Press writers Eric Tucker in Washington, Terrance Harris and Jason Dearen in Orlando and photographer Alan Diaz in Fort Pierce, Florida, contributed to this report.