Omar Mateen, a 29-year-old U.S. citizen, opened fired in a gay nightclub in Orlando, Florida, early on Sunday, killing 49 and injuring 53.

Mateen, who had been armed with a handgun and AR-15-type rifle, was killed at the scene after a SWAT team stormed the club, Hopper said. But what led him to open fire inside nightclub is something that investigators are continuing to piece together.

Mateen was born and raised in New York until his family moved to Port St. Lucie in eastern Florida. He was of Afghan descent.

He had worked as a contracted security guard in Florida since 2007 for G4 Security — a company that provides security for federal buildings.

G4 Security said in a statement Monday that when Matteen was hired he was given a "detailed company screening when he was recruited in 2007 and re-screened in 2013 with no adverse findings."

Before becoming a security guard, he worked for the Florida Department of Corrections. Mateen worked at Martin Correctional Institution from October 2006 to April 2007, a department spokesman told BuzzFeed News.

In 2009, he married Sitora Yusufiy in New Jersey, but she left him after a few months of marriage when he began physically abusing her, she told reporters Sunday.

The divorce wasn't finalized until 2011, government records obtained by BuzzFeed News show. Yusufiy said it took a year and a half for the divorce to go through because she was in New Jersey, while Mateen was in Florida.

Yusufiy met Mateen online and, in the beginning, acted normal, but after a few months, "I saw he was bipolar," she said, adding that he "would get mad out of nowhere." She also called him "mentally ill."

Yusufiy also told reporters Mateen was pursuing a career as a police officer at the time and that she believed he had worked at a Fort Pierce-based juvenile detention center.

Mateen's father, Seddique Mateen, told NBC that a few months ago Mateen was angered by two men kissing in Miami, especially because it took place in front of his three-year-old son.

His father has since held his own news conference, telling reporters that the family was totally unaware of any issues.

"We are saying we are apologizing for the whole incident," Seddique Mateen said. "We weren't aware of any action he is taking. We are in shock like the whole country."