This is a developing news story. Check back for updates.
What we know
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 Omar Mateen of Port St. Lucie, Florida. His father, Mir Seddique, told NBC News that his son got angry when he recently 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.
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 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.
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."
People who were inside the gay Florida nightclub where 50 people were killed are describing a scene of mass chaos.
Two men who were in Club Pulse discussed the Sunday massacre in Facebook chats with The Associated Press.
Orlando resident Brand White was shot. He says, "We are dancing and all of a sudden it just started like a rolling thunder, loud and everything went black."
The 30-year-old was with his cousin, who mentioned something about "a guy with a bomb." After that, White says, his memory is fuzzy; he said he doesn't recall leaving the club or who took him to the hospital.
White was shot in the shoulder. He spoke to The AP as he was being monitored at a hospital. He said he received a blood transfusion.
His cousin was unaccounted for — no one had heard from him as of 2:45 p.m.
Another man, Brett Rigas, says he and his partner were dancing when they heard shots. Rigas was shot in the arm and hid behind a bar. About five minutes later, authorities came in and told everyone to put their hands up and run out.
Rigas said he saw bodies as he ran out.
The suspected Orlando nightclub gunman had been licensed as a private security officer in Florida.
State records show suspected shooter Omar Mateen held the firearms license since at least 2011. It was set to expire in September 2017.
It wasn't immediately clear where, if anywhere, Mateen had worked as a security officer. An armed guard license in Florida requires 28 hours of classroom training by a licensed instructor.
Attorney General Loretta Lynch is cutting short a visit to Beijing for cybersecurity meetings with Chinese officials and returning to the United States to monitor developments in the nightclub shooting investigation.
Lynch says the Justice Department, including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, is supporting the investigation.
She says in a statement that she's gotten updates from Deputy Attorney General Sally Yates and FBI Director James Comey.
Prime Minister Benjamin Netanyahu says Israel stands "shoulder to shoulder" with the United States after the shooting at a gay nightclub in Orlando that killed 50 people and wounded dozens more.
Netanyahu said Sunday that "on behalf of the people and government of Israel, I extend our deepest condolences to the American people following last night's horrific attack on the LGBT community in Orlando."
He wished "heartfelt sympathies to the families of the victims" and "full and speedy recovery to the wounded."
The Orlando attack dominated news in Israel, which has seen a wave of Palestinian attacks in recent months. On Wednesday two Palestinian gunmen killed four people and wounded five others in Tel Aviv.
LGBT groups in Israel planned rallies and other support for the community in Orlando.
Police departments across the country are increasing patrols near locations frequented by the LGBT community after a gunman killed 50 people at a gay nightclub in Orlando.
In Los Angeles, the mayor says a heavily armed person who was headed to a gay pride parade had been arrested by Santa Monica police. Mayor Eric Garcetti says the arrest was completely unrelated to the Orlando nightclub shooting.
Officer Ernesto Rodriguez of Miami Beach Police Department says the agency is saddened by the massacre and out of an abundance of caution will step up patrols. Boston Pride organizers plan to hold a moment of silence at this weekend's scheduled block parties to honor the Orlando victims and police there said there will be a heavier presence at those events.
The Baltimore Police Department says it is reaching out to the city's LBGT community to discuss concerns and safety after the Orlando massacre.
President Barack Obama says the worst mass shooting in U.S. history is a further reminder of how easy it is for someone to get a weapon that allows them to shoot people in a school, in a house of worship, a movie theater or a gay nightclub.
Speaking from the White House, Obama says the United States has to decide if that is the "country we want to be." He says that doing nothing is a decision as well.
The shooting has thrust the topic of gun control back into focus as a presidential election nears.
Presumptive Democratic nominee Hillary Clinton has called for expanding background checks to sales at gun shows and online purchases, and for reinstating a ban on assault weapons. Presumptive Republican nominee Donald Trump has said the existing background check system should be fixed, not expanded, and that assault-weapons bans do not work.
President Barack Obama has ordered flags to be flown at half-staff at the White House and federal buildings until sunset Thursday "as a mark of respect for victims of the act of hatred and terror" at a gay Florida nightclub.
He's also directing the same observance at embassies and other U.S. government facilities abroad.
Obama addressed the nation Sunday, calling the shooting "an act of terror" and an "act of hate."
Fifty were killed, including the shooter, and 53 more hospitalized.
Donald Trump isn't pausing his political commentary for the biggest mass shooting in U.S. history unfolding in Florida.
It was unclear whether the shooter who killed at least 50 people in an Orlando, Florida, nightclub was associated with a radical religious organization. President Barack Obama addressed the nation, calling the shooting "an act of terror" and an "act of hate."
Trump tweeted as Obama began speaking: "Is President Obama going to finally mention the words radical Islamic terrorism? If he doesn't he should immediately resign in disgrace!"
A law enforcement official tells The Associated Press that the shooter in the massacre at a gay Florida nightclub was known to the FBI before the incident and had been looked at by agents within the last few years.
The official spoke to The Associated Press Sunday about the shooting, the deadliest in U.S. history. The official was not authorized to discuss the ongoing investigation and on spoke on condition of anonymity.
— AP Writer Eric Tucker in Washington
President Barack Obama says the nightclub shooting in Orlando was an "act of terror" and an "act of hate."
He said Sunday that the FBI is investigating it as an act of terrorism and that no effort will be spared to determine whether the shooter was affiliated with terrorist groups.
Obama is speaking at the White House after 50 people were killed overnight at a nightclub in Florida. Officials have said 53 more are hospitalized.
Obama is noting that the killer targeted a gay nightclub. He says it's a "sobering reminder" that an attack on any American is an attack "on all of us."
PREVIOUS STORY FOLLOWS:
ORLANDO, Fla. (AP) — A gunman wielding an assault-type rifle and a handgun opened fire inside a crowded gay nightclub early Sunday, killing at least 50 people before dying in a gunfight with SWAT officers, police said. It was the deadliest mass shooting in American history.
Authorities were investigating the attack on the Florida dance hall as an act of terrorism. The gunman's father recalled that his son recently got angry when he saw two men kissing in Miami and said that might be related to the assault.
At least 53 people were hospitalized, most in critical condition, officials said. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.
Mayor Buddy Dyer said all of the dead were killed with the assault rifle.
"There's blood everywhere," Dyer said.
Witnesses described a chaotic scene when the gunfire began shortly before the club known as Pulse was to close.
"Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance," said Jackie Smith, who had two friends next to her get shot. "I just tried to get out of there."
The suspect exchanged gunfire with a police officer working at the club, which had more than 300 people inside. The gunman then went back inside and took hostages, Police Chief John Mina said.
Around 5 a.m., authorities sent in a SWAT team to rescue the hostages.
Authorities were looking into whether the attack was an act of domestic or international terror, and if the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.
"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County Sheriff Jerry Demings said.
The previous deadliest mass shooting in the U.S. was the 2007 attack at Virginia Tech, where a student killed 32 people before killing himself.
The suspect in the Orlando attack was identified as Omar Mateen of Port St. Lucie, Florida.
Rep. Alan Grayson named the shooter, citing law enforcement officials. A federal law enforcement official with knowledge of the investigation also confirmed the name. The official was not authorized to discuss the investigation publicly and spoke on condition of anonymity.
Mateen's father, Mir Seddique, told NBC News about his son seeing the men kissing a couple of months ago.
"We are saying we are apologizing for the whole incident," Seddique said. "We are in shock like the whole country."
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 towards that."
Mateen's father said the attack had nothing to do with religion, he said.
In addition to the guns, the shooter also had some sort of "suspicious device," the police chief said.
Authorities said they had secured the suspect's vehicle, a van, outside the club.
A SWAT truck and a bomb-disposal unit were on the scene of an address associated with Mateen in a residential neighborhood of Fort Pierce, Florida, about 118 miles southeast of Orlando.
Relatives and friends, many in tears, gathered outside the hospital to learn the fate of loved ones.
Smith did not know the conditions of her friends and came out of the hospital and burst into tears.
Christine Leinonen drove to Orlando at 4 a.m. after learning of the shooting from a friend of her 32-year-old son, Christopher Leinonen, who was at Pulse and is missing.
She has not heard from her son and fears the worse.
"These are nonsensical killings of our children," she said, sobbing. "They're killing our babies!"
She said her son's friend Brandon Wolf survived by hiding in a bathroom and running out as the bullets flew.
A woman who was outside the club early Sunday was trying to contact her 30-year-old son, Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: "He's coming."
"The next text said: 'He has us, and he's in here with us,'" Mina Justice said. "That was the last conversation."
Pulse posted on its own Facebook page around 2 a.m.: "Everyone get out of Pulse and keep running." Just before 6 a.m., the club posted an update: "As soon as we have any information, we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love."
President Barack Obama was briefed on the attack and asked for regular updates on the investigation, the White House said.
Jon Alamo said he was at the back of one of the club's rooms when a man holding a weapon came into the front of the room.
"I heard 20, 40, 50 shots," Alamo said. "The music stopped."
Club-goer Rob Rick said the shooting started just as "everybody was drinking their last sip."
He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area where only workers are allowed. People inside were then able to escape through the back of the club.
Christopher Hansen said he was in the VIP lounge when he heard gunshots. He continued to hear shooting even after he emerged and saw the wounded being tended across the street.
"I was thinking, 'Are you kidding me?' So I just dropped down. I just said, 'Please, please, please, I want to make it out,'" he said. "And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot."
The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who had been a YouTube sensation and former contestant on "The Voice." She was killed after a concert in Orlando by a 27-year-old man who later killed himself.
Associated Press writers Eric Tucker in Washington, Terrance Harris and Jason Dearen in Orlando and photographer Alan Diaz in Fort Pierce, Florida, contributed to this report.
This story has been corrected to show that Banks is with the Florida Department of Law Enforcement, not the FBI.
Stay with the Times Free Press for updates on this developing news story.