What We Know: Nightclub Shooting Deadliest On US Soil

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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 Port St. Lucie, Florida.

In a 911 call from the club, Mateen professed allegiance to the leader of the Islamic State, Abu Bakr al-Baghdadi.

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.

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

Security company G4S said in a statement to the Palm Beach Post that he had been an employee of the company since September 10, 2007.

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: A former Afghan official says Seddique Mir Mateen appears on a television program known for "its anti-U.S. tirades" and "pro-Taliban" remarks. The official says the program is broadcast in the Dari language. 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."