Omar Mateen, 29, of Fort Pierce, Florida, carried an assault rifle and a pistol into the packed Pulse club about 2 a.m. Sunday and started shooting.
In addition to the people killed, he wounded at least 53 others, police said.
During the attack, Mateen called 911 to pledge allegiance to the ISIS terror group and mentioned the Boston Marathon bombers, according to a U.S. official.
After a standoff of about three hours, while people trapped inside the club desperately called and messaged friends and relatives, police crashed into the building with an armored vehicle and stun grenades. They killed Mateen after the rampage -- the deadliest terror attack in the United States since 9/11
"It appears he was organized and well-prepared," Orlando Police Chief John Mina said. Authorities said they haven't found any accomplices.
'An act of hate'
ISIS sympathizers have reacted by praising the attack on pro-Islamic State forums.
"We know enough to say this was an act of terror and act of hate," President Obama said in an address to the nation from the White House.
While the violence could have hit any American community, "This is an especially heartbreaking day for our friends who are lesbian, gay, bisexual or transgender," he said.
Omar Mir Seddique Mateen was born in 1986 in New York. Most recently he lived in Fort Pierce, about 120 miles southeast of Orlando. Fearing explosives, police evacuated about 200 people from the apartment complex where he lived while they looked through his residence for evidence.
Mateen's parents, who are from Afghanistan, said he'd expressed outrage after seeing two men kiss in Miami, but they didn't consider him particularly religious and didn't know of any connection he had to ISIS.
He was married in 2009 to a woman originally from Uzbekistan, according to the marriage license, but he filed documents to end the marriage in 2011.
Sitora Yusufiy, interviewed by CNN in Boulder, Colorado, said she and Mateen were together about four months, though it took a long time to complete the divorce because they lived in different parts of the country after separating.
Mateen was a normal husband at the beginning of their marriage but started abusing her after a few months, she said. She said Mateen was bipolar, although he was not formally diagnosed. She also said Mateen had a history with steroids. He was religious but she said she doesn't think his religion played in to the attack.
According to a mortgage document, Mateen remarried a woman named Noor Salman and they lived in a Fort Pierce condo. CNN's Sara Sidner talked to a source in Rodeo, California, who knows the family well. That source said Salman's mother wept and tearfully wondered "why is God doing this to me?"
Mateen had worked since 2007 as a security officer at G4S Secure Solutions
, one of the world's largest private security companies.
A message posted in Arabic on a dark web site associated with the ISIS news agency Amaq said "the armed attack that targeted a gay night club in the city of Orlando in the American state of Florida and that bore more than a 100 killed and wounded was carried out by an Islamic state fighter."
But CNN's Salma Abdelaziz, who translated the message and closely monitors ISIS messaging, cautioned about taking the message at face value.
She said the language is inconsistent with previous ISIS announcements and that the Arabic word for gay was used rather than an epithet normally used by ISIS. Also, there was no claim that the attack was directed, just an after-the-fact claim the gunman was an ISIS fighter, she said.
FBI Assistant Special Agent Ronald Hopper said the agency was aware of Mateen. The FBI interviewed him in 2013 and 2014 after he expressed sympathy for a suicide bomber, Hopper said.
"Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going," Hopper said.
Mateen was not under investigation at the time of Sunday's shooting and was not under surveillance, Hopper said.
In the past two weeks, Mateen legally purchased a Glock pistol and a long gun, ATF Assistant Special Agent in Charge Trevor Velinor told reporters.
It's not known if those weapons were used in the attack.
"He is not a prohibited person. They can legally walk into a gun dealership and acquire and purchase firearms. He did so. And he did so within the last week or so," Velinor said.
Scene inside the club
Pulse describes itself
as "the hottest gay bar" in the heart of Orlando. Hours before the shooting, the club urged partygoers to attend its Latin event Saturday night. The club is a vast, open space that was hosting more than 300 patrons late Saturday and into Sunday morning.
People inside the nightclub described a scene of panic made more confusing by the loud music and darkness.
"At first it sounded like it was part of the show because there was an event going on and we were all having a good time," clubgoer Andy Moss said. "But once people started screaming and shots just keep ringing out, you know that it's not a show anymore."
Christopher Hansen said he was getting a drink at the bar about 2 a.m. when he "just saw bodies going down." He heard gunshots, "just one after another after another."
The gunshots went on for so long that the shooting "could have lasted a whole song," he said.
When the shots erupted, Hansen hit the ground, crawling on his elbows and knees, before he spotted a man who had been shot.
"I took my bandana off and shoved it in the hole in his back," Hansen said, adding that he saw another woman who appeared to be shot in the arm.