Omar Mateen passed a psychological test required by his employer, security company G4S, when he was hired in 2007. Credit: Balkis Press/Sipa USA via AP Images
Undated photo or selfie of Omar Mateen, identified as the gunman in mass shooting at a gay club in Orlando, Florida on June 12, 2016. The shooting death toll rose to 50 with a further 53 wounded.
Omar Mateen, identified as the gunman in the worst mass shooting in U.S. history, had been investigated by the FBI and had an alleged history of domestic violence.Credit: Balkis Press/Sipa USA via AP Images 

UPDATE, June 13, 2016: This post has been updated to reflect that Omar Mateen worked for G4S. The number of people killed in the shooting also has been updated.

The worst mass shooting in U.S. history was carried out by a security guard licensed to carry a firearm.

Authorities have identified Omar Mateen, 29, as the gunman who left 49 people dead and more than 50 others wounded early Sunday morning at Pulse, a popular gay nightclub in Orlando, Florida.

Mateen had been investigated by the FBI and had an alleged history of domestic violence against an ex-wife, according to news reports.

His licenses to work as an armed guard were active at the time of the shooting. The Bureau of Alcohol, Tobacco, Firearms and Explosives confirmed Sunday that he recently had purchased two firearms legally.

Reveal originally checked Mateen’s arrest record in 2014 as part of an investigation into the armed security guard industry, which is regulated differently in each state. In Florida, authorities conduct criminal background checks before issuing guard licenses.

Mateen did not have a criminal record at the time. But reporters found that many guards were able to slip through the cracks of state licensing systems, including in Florida, where armed guards are not subject to mental health screenings before receiving licenses to carry firearms on the job. Reveal found many guards with histories of substance abuse who were allowed to work as armed guards.

It is unclear if Mateen had any other issues in his record, such as a record of a mental health commitment or substance abuse problems, that would have prohibited him from owning or using a gun. His ex-wife told The Washington Post that he was mentally unstable and violent.

“He was not a stable person,” she told the paper. “He beat me. He would just come home and start beating me up because the laundry wasn’t finished or something like that.”

Mateen had worked for G4S, one of the largest security companies in the United States. An in-house newsletter from 2012 identified an Omar Mateen of West Palm Beach as a guard for the company. His ex-wife told The Washington Post that he worked as a guard at a nearby facility for juvenile delinquents.

Orlando was still reeling from a very public recent killing. Christina Grimmie, who rose to fame as a contestant on NBC’s “The Voice,” was shot to death while signing autographs there Friday.

Julia B. Chan contributed to this report. Shoshana Walter can be reached at Follow her on Twitter: @shoeshine.

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Shoshana Walter was a senior reporter and producer at Reveal, covering the criminal justice and child welfare systems. She's working on a book for Simon & Schuster about the failures of our country's addiction treatment system. At Reveal, she reported on exploitative drug rehab programs that require participants to work without pay, armed security guards, and sex abuse and trafficking in the marijuana industry. Her reporting has prompted new laws, numerous class-action lawsuits and government investigations. Her stories have been named finalists for the Pulitzer Prize, Selden Ring and National Magazine Awards. She has also been honored with the Livingston Award for National Reporting, the IRE medal, the Edward R. Murrow award, the Knight Award for Public Service, a Loeb Award and Sigma Delta Chi Award for investigative reporting. Her Reveal podcast, "American Rehab," was named one of the best podcasts of the year by The New Yorker and The Atlantic and prompted a congressional investigation.

Walter began her career as a police reporter for The Ledger in Lakeland, Florida, and previously covered violent crime and the politics of policing in Oakland, California, for The Bay Citizen. A graduate of Mount Holyoke College, she has been a Dart Center Ochberg fellow for journalism and trauma at the Columbia University Graduate School of Journalism and a John Jay/Harry Frank Guggenheim fellow in criminal justice journalism. She is a fellow with the Watchdog Writers Group at the University of Missouri School of Journalism and is based in Oakland, California.