Omar Mateen, a 29-year-old U.S.-born citizen from Port St. Lucie, Fla., has been identified as the suspect in Sunday's mass shooting that left at least 50 dead and more than 50 others wounded at Pulse, a "gay" nightclub in Orlando, Florida.

According to public records, Mateen had a permit to carry a concealed weaponed and was a licensed security guard. He had worked for G4S, a security company.

Mateen was a registered Democrat and records show he was married to a New Jersey woman originally from Uzbekistan in Port St. Lucie in 2009.

According to Florida court records, the two formally divorced in 2011.

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“He was not a stable person,” said the ex-wife, who spoke on the condition of anonymity to the Washington Post. “He beat me. He would just come home and start beating me up because the laundry wasn’t finished or something like that.”

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His ex-wife said at one point, Mateen worked as a guard at a nearby facility for juvenile delinquents.

Staff at the Islamic Center, a mosque in Ft. Pierce, Fla., confirmed to CBS News that Mateen has worshiped at the facility regularly for several years.

The mosque condemned the attack in a statement released Sunday afternoon:

"We condemn this monstrous attack and offer our heartfelt condolences to the families and loved ones of all those killed or injured. The Muslim community of Fort Pierce joins our fellow Americans in repudiating anyone or any group that would claim to justify or excuse such an appalling act of violence."

A co-worker, who worked with Mateen at G4S Security for several months in 2014 and 2015, described him as homophobic, "unhinged and unstable," according to Florida Today.

Daniel Gilroy said Mateen began sending him multiple texts a day, leading Gilroy to quit his job.

"I quit because everything he said was toxic," Gilroy said. "And the company wouldn't do anything. This guy was unhinged and unstable. He talked of killing people."

Facebook posts from Mateen's former classmates at Martin County High School in Stuart, Fla., describe how Mateen made anti-American statements and seemed unsympathetic about the loss of life following the Sept. 11, 2001 terror attacks.

Mateen, who was born in New York to Afghan parents, came to the attention of federal authorities in 2013 and 2014, a senior law-enforcement source told the Daily Beast.

The FBI at one point opened an investigation into Mateen but subsequently closed the case.

“He’s a known quantity,” the source told the Daily Beast. “He’s been on the radar before.”

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CNN anchor Jake Tapper said during a live broadcast that the network would no longer identify Mateen during news coverage of the shooting aftermath: "We are now learning more about the suspect, learning more about him, let us use the information that I'm about to share with you sparingly, his name was Omar Mateen, that is a name I will not mention again for the rest of the show. And that is his photograph, we know he was training as a private security guard … let us take that graphic down."

Rep. Adam Schiff, D-Calif., told CNN he was informed by local police that Mateen pledged allegiance to ISIS, but could not confirm when the gunman made the declaration.

Sen. Bill Nelson, D-Fla., said Senate Intelligence staff had information that there was an ISIS connection to the shooting.

NBC News later confirmed from law-enforcement officials that Mateen had called 9-1-1 moments before the shooting to pledge allegiance to the leader of ISIS.

Mateen also referenced the Tsarnaev brothers, who carried out the Boston Marathon bombings in 2013, at the scene, sources said.

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Mateen's father, Omar Mir Seddique Mateen, told NBC News, "This has nothing to do with religion."

He said the sight of two men kissing a few months ago in Miami angered his son, and speculates that could have triggered his decision to kill.

“We are saying we are apologizing for the whole incident,” the father added. “We weren’t aware of any action he is taking. We are in shock like the whole country.”

Mateen's friend, who asked not to be identified, told the Washington Post that several years ago Mateen went on the pilgrimage to Mecca in Saudi Arabia.

“He was quite religious,” the friend said.

Federal officials told NBC News that Mateen went to Saudi Arabia in 2011 and 2012.

A spokesman for Saudi Arabia's Ministry of the Interior told NBC News that Mateen visited to perform a pilgrimage to Mecca.

Dartevious Parker, a neighbor who lived near Mateen, said Mateen was living with a woman at the time of the massacre and had a 3-year-old son.

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Photographs of Mateen on social media show him wearing NYPD shirts, but law enforcement said the killer had no association with the department and the unofficial garments can be purchased by the general public in retail stores, according to TMZ.

Police searched Mateen's Florida home shortly after the massacre.

In 2007, Mateen graduated with an associate degree of science from Indian River State College in Fort Pierce.

Mateen, who was armed with a rifle and a handgun, entered Pulse club around 2 a.m. Sunday morning and began shooting, as WND reported.

The deadliest single-person mass murder in American history before Orlando occured in Bath, Michigan, in 1927, when a man bombed a school, killing 44 people.