The Orlando shooting: What we know and don't know

A gunman who claimed allegiance to the Islamic State killed 50 people and wounded 53 more when he opened fire in a crowded gay nightclub in Orlando, Florida, early Sunday. It was the worst mass shooting in American history. Here is the latest:

Who Is the Suspect?

— The gunman was Omar Mateen, 29, a U.S. citizen whose parents were from Afghanistan. He claimed allegiance to the Islamic State in a 911 call he made at the time of the attack, law enforcement officials said. He lived in Fort Pierce, Florida.

— The Orlando Police Department said Mateen was born in New York. Court records indicate he was married and divorced.

— The FBI investigated Mateen for possible terrorist ties in 2013 and 2014 but did not believe him to be a threat. The 2014 investigation centered on a possible link between Mateen and Moner Mohammad Abusalha, an American from Florida who became a suicide bomber for an extremist group in Syria.

— The global security company G4S, based in Britain, said Mateen had worked for it as a guard since 2007. A co-worker said he had repeatedly complained to the company that Mateen used racial, ethnic and sexist slurs and talked about killing people.

— The Bureau of Alcohol, Tobacco, Firearms and Explosives said Mateen had legally bought both weapons used in the attack, a handgun and a long gun, in Florida within the last week.

What Happened?

— Mateen opened fire inside the Pulse nightclub in Orlando, a popular gay club, at about 2 a.m. He was armed with an AR-15-style assault rifle and a handgun.

— Mateen shot about one-third of the people in the packed club. Hundreds of panicked clubgoers escaped and fled into the streets.

— Mateen holed up inside the club during the attack and effectively held dozens of people hostage. Some of them hid in a restroom and frantically texted friends and family for help.

— Mateen was killed by a police SWAT team when it raided the building at about 5 a.m. with an armored vehicle and stun grenades. One police officer was wounded, and at least 30 people were rescued.

Who Were the Victims?

— The city of Orlando set up a special web page where it said it would release the names of victims. It has released eight names: Stanley Almodovar III, Peter O. Gonzalez-Cruz, Juan Ramon Guerrero, Kimberly Morris, Luis Omar Ocasio-Capo, Eric Ivan Ortiz-Rivera, Edward Sotomayor Jr., and Luis S. Vielma.

How Does This Compare With Other Mass Shootings?

— More people were killed in Orlando than in any previous mass shooting in the United States. The 2007 shooting at Virginia Tech killed 32 people, while 26 people were killed in the 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut.

— This is the second mass shooting in the United States linked to sympathizers of the Islamic State since December, when a married couple killed 14 people in a rampage in San Bernardino, California. The Orlando shooting was the worst terrorist attack on American soil since Sept. 11, 2001.

— President Barack Obama called the attack "an act of terror and an act of hate" and said the U.S. flag would be flown at half-staff at the White House, embassies and military facilities. "In the face of hate and violence, we will love one another," he said.

— The Islamic State claimed responsibility for the massacre in a statement released over an encrypted phone app. The group said the attack "was carried out by an Islamic State fighter," according to a transcript provided by the SITE Intelligence Group, which tracks jihadi propaganda.

What We Don't Know

— Law enforcement officials have not said whether Mateen had any direct link to the Islamic State, such as training or any sort of direct communication. The statement by the Islamic State did not provide details about its relationship with Mateen.

— It is possible he was "self-radicalized" — that is, claimed allegiance to the Islamic State but had no direct tie — like the husband-and-wife team behind the San Bernardino attack last year.

— The authorities have not released the names of most of the victims.

— The police in Santa Monica, California, arrested a heavily armed man who said he was in the area for the gay pride parade in West Hollywood, but they did not know of a connection between him and the Orlando attack.