The man accused of killing 50 people and injuring another 53 at a Florida nightclub early Sunday called 911 before the shooting to pledge allegiance to the leader of ISIS, NBC News is reporting.

The shooting at Pulse, a gay nightclub in Orlando, Fla., is the deadliest shooting spree in U.S. history.

Various law enforcement officials have identified the shooter as 29-year-old Omar Mateen of Port St. Lucie, Fla. Mateed was fatally shot by police.

Mateen appeared to be a follower of ISIS propaganda and referenced the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, at the scene, sources told NBC.

Behind the Bloodshed: The untold story of America's mass shootings

In a Sunday afternoon news conference, FBI officials confirmed Mateen as the deceased suspect. Mateen was an American citizen born in New York.

The FBI questioned Mateen twice in 2013 after comments he allegedly made. In 2014, he was questioned in regard to possible connections to Moner Mohammad Abusalha, who was behind a suicide bombing in Syria. Those investigations were closed.

ATF officials said that Mateen legally purchased firearms in the last several days.

According to NBC News, Mateed's family believes he may have been pushed over the edge by hate against the LGBT community.

"We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry," Mir Seddique, told NBC News on Sunday. "They were kissing each other and touching each other and he said, 'Look at that. In front of my son they are doing that.' And then we were in the men's bathroom and men were kissing each other."

Multiple media outlets have said that Mateen is Muslim. His father said the attack "had nothing to do with religion."

In 2006, he filed a petition to change his name from Omar Mir Seddique to Omar Mir Seddique Mateen, according to NBC News.

Seddique said Mateen was a husband and father to a 3-year-old son, NBC reports. He worked in security and had attended Indian River State College, where he got an associate's degree in criminal justice, his father told NBC.

Sources tell NBC News' Tom Winter that Mateen also referenced the Boston Marathon bombing suspects during the shooting rampage. Winter reports that Mateen had a valid security guard's license and a valid firearms license at the time of the shooting according to public records.

Material in this report was used from NBC News and USA Today.