A gunman opened fire at a gay nightclub in Orlando, Florida, early Sunday, killing 50 people and wounding 53 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 got angry a couple of months ago when he saw two gay men kissing. Here’s what’s known 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 is likely to climb.

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, according to a law enforcement official familiar with the investigation who was not authorized to discuss the matter publicly and spoke on condition of anonymity.

The FBI says Mateen was a U.S. citizen and not under surveillance. In 2014, authorities discovered he had ties to an American suicide bomber, but they said 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. Mateen legally purchased at least two firearms within the past week or so, said Trevor Velinor, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives.

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.

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.”