Terry DeCarlo, executive director of the LGBT Center of Central Florida, center, is comforted by Orlando City Commissioner Patty Sheehan, right, after a shooting involving multiple fatalities at a nightclub in Orlando, Fla., Sunday, June 12, 2016. (AP Photo/Phelan M. Ebenhack)

A gunman opened fire at a gay night club in Orlando, Florida, early Sunday, leaving 50 people dead and 53 wounded before he was killed in a shootout with SWAT team members. Here's what we know about the deadliest mass shooting in modern U.S. history:

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MASS CASUALTIES: At least 50 people were killed, and 53 were hospitalized, most in critical condition, officials said. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.

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THE SHOOTER: 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, according to a law enforcement official.

Mateen was a U.S. citizen born in New York. His ex-wife said his family was from Afghanistan.

Authorities say Mateen was not under surveillance, but that in 2014, they discovered he had ties to an American suicide bomber. They said the ties were minimal and they didn't think he represented a threat at the time.

They say he also made inflammatory comments to co-workers in 2013. He worked for the GS4 global security company.

Authorities also say Mateen legally purchased at least two firearms within the past week or so.

Mateen's father, Seddique Mir Mateen, said his son got angry when he saw two men kissing in Miami a couple of months ago. He said that might be related to the attack. The father said the attack had nothing to do with religion.

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FATHER'S ACTIVITIES:

Seddique Mir Mateen is a life insurance salesman who started a group in 2010 called Durand Jirga, Inc., according to Qasim Tarin, a businessman from California who was a Durand Jirga board member. The name refers to the Durand line, the long disputed border between Afghanistan and Pakistan.

Tarin said Seddique Mir Mateen had a television show on which they discussed issues facing Afghanistan. He said Seddique Mir Mateen loves the U.S.

But a former Afghan official said the "Durand Jirga Show" expresses support for the Taliban, has an anti-Pakistan slant, complains about foreigners in Afghanistan and criticizes U.S. actions there. The official spoke only on condition of anonymity because he did not want to be linked to coverage of the shooting.

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WHAT HAPPENED: Police say Mateen, equipped with an assault rifle and a handgun, opened fire on patrons early Sunday. He also exchanged fire with an officer working at the club about 2 a.m., when more than 300 people were inside. Police say the gunman then went back inside and took hostages. Police sent in a SWAT team to rescue hostages about 5 a.m. and Mateen died in an exchange of gunfire with SWAT officers.

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THE VICTIMS:

The city of Orlando is publishing the names of those killed on its website after their families have been contacted. The list can be found at http://www.cityoforlando.net/blog/victims/

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SECOND DEADLY SHOOTING IN TWO DAYS: The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself. Grimmie was a YouTube sensation and former contestant on "The Voice."

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This story has been corrected to reflect that it was an ATF agent, not FBI agent, who said that shooter legally purchased firearms, and to correct father's name to Seddique Mir Mateen, instead of Mir Seddique.

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This story has been corrected to reflect that shooter was from Fort Pierce, Florida, not Port St. Lucie.

News from © The Associated Press, 2016