Phelan M. Ebenhack, Associated Press
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.

A gunman opened fire at a gay night club in Orlando, Florida, early Sunday, killing 50 people and wounding 53 more before he was killed in a shootout with SWAT team members. Authorities say he may have had a connection with radical Islamic terrorism, and his father said he became angry a couple of months ago when he saw two gay men kissing. Here's what we know about the deadliest mass shooting in U.S. history:

MASS CASUALTIES: At least 50 people are dead, 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.

THE SHOOTER: Authorities have identified the shooter as 29-year-old Omar Mateen of Port St. Lucie, Florida. They say that conversations he had with police in a 911 call the night of the shooting involved the Islamic State. They say he was a U.S. citizen and was not under surveillance. In 2014, they discovered he had ties to an American suicide bomber, but they were minimal, and he didn't constitute a threat at the time. They say he also made inflammatory comments to co-workers in 2013. Ronald Hopper of the FBI says Mateen legally purchased at least two firearms within the last week or so. Mateen's father, Mir Seddique, 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. Seddique said the attack had nothing to do with religion. Mateen's ex-wife said his family was from Afghanistan, but that he was born in New York.

MORE ON THE SHOOTER:

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.

ACT OF TERROR?: Orange County Sheriff Jerry Demings called the attack a "domestic terror incident." Other authorities said they are looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone. 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 toward that."

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.

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/

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."