The massacre is the worst mass shooting in the history of the United States.
Officials said a hostage situation developed after the gunman stormed the Pulse Nightclub about 2 a.m.
The shooter was identified by several law enforcement sources as Omar Mateen, 29.
He was shot dead about three hours later when a SWAT team entered the club, police said. A handgun and AR-15-type rifle were recovered at the scene, according to police.
The law enforcement sources told NBC News that Mateen was born in New York in 1986 and was listed as living at a residence in Port St. Lucie, about 125 miles south of Orlando.
Mateen had active security officer and firearm licenses, according to Florida records, and his family said he worked in security. Marriage records show he was married in Port St. Lucie in 2009, and a relative said he had a 3-year-old son.
The incident is being investigated as an act of terrorism, officials said.
Mir Seddique, Mateen's father told NBC News, "this has nothing to do with religion." Seddique said his son got angry when he saw two men kissing in Miami a couple of months ago and thinks that may be related to the shooting.
"We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," Seddique said.
Meanwhile, a man who answered the phone at Mateen's address, Mustafa Abasin, told NBC News: "We are in shock and we are sad." He would not say how he knew Mateen, but said he was helping investigators.
Rep. Alan Grayson, D-Fla., whose district includes the area of the massacre, said the attack was "more likely than not ideologically motivated."
"It's no coincidence that the attack took place where it did and where it did," he said. "It might be that we've seen the commission of an awful hate crime."
Grayson said investigators were searching Mateen's home and combing the nightclub. Processing the gruesome scene would "take hours," Grayson said. "There is blood everywhere."
Police said Mateen was a U.S. citizen, but some of his family members are not. They would not say where those family members were from, and while they have not confirmed that Mateen was Muslim, several Muslim groups rebuked the attack.
Note EU-Digest: If it happens to turn out to be an ideologically motivated terrorist attack it shows once again the cowardliness of these deranged killers. This massacre also highlights the urgent need for gun control in America and better screening of security personnel before putting lethal weapons into their hands. Either way - we can only hope that the investigation into this hideous crime comes up with answers very soon.
Read more: Orlando Nightclub Shooting: Mass Casualties After Gunman Opens Fire in Gay Club - NBC News