FORT PIERCE, Fla. — At least four regular customers at the gay Orlando nightclub where a gunman killed 49 people Sunday morning said Monday that they believe they had seen the killer, Omar Mateen, there before.

“Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent,” said Ty Smith, who also uses the name Aries.

He saw Mateen at the club at least a dozen times, he told the Orlando Sentinel.

“We didn’t really talk to him a lot, but I remember him saying things about his dad at times,” Smith said. “He told us he had a wife and child.”

He worked as a security guard at a gated retirement community in South Florida, according to his employer, G4S, a private security firm based in Jupiter.

Before that, he was assigned, for a time, as a security guard at the St. Lucie County Courthouse in Fort Pierce, according to county Administrator Thomas Genung.

Joseph Abreu, spokesman for the St. Lucie County clerk of courts, said Mateen last used his security badge in May 2013. That’s the same year the FBI investigated “inflammatory” comments he made to coworkers, claiming ties to terrorists.

“I think the important part for us to know is, yes, he had access, and in hindsight, looking back, we could see all these preventable measures we could have taken,” Abreu said.

It is unclear when Mateen was first assigned to the courthouse, Abreu said.

In a prepared statement, G4S said: “We are deeply shocked by this tragic event. We can confirm that Omar Mateen had been employed by G4S since September 10th, 2007.

“Mateen underwent company screening and background checks when he was recruited in 2007 and the check revealed nothing of concern. His screening was repeated in 2013 with no findings,” according to the statement.

“In 2013, we learned that Mateen had been questioned by the FBI but that the inquiries were subsequently closed. We were not made aware of any alleged connections between Mateen and terrorist activities, and were unaware of any further FBI investigations.”

The FBI interviewed him three times, followed him and listened in on his phone calls after that 2013 complaint, according to FBI Director James Comey, but after 10 months, agents closed the file, concluding that he was not a threat.

Two months later, in early 2014, the FBI investigated again after it found out he knew a suicide bomber in Syria but ended the inquiry after a witness said that Mateen had gotten remarried, had a child and changed, Comey said.

Mateen worked at a state prison, Martin Correctional Institution in Indiantown, for six months, starting Oct. 27, 2006, according to Florida Department of Corrections spokesman Alberto Moscoso.

Other information about his assignment and the circumstances under which he left weren’t available.

In a news conference, Mateen’s ex-wife, Sitora Yusufiy, described him as seriously mentally ill, according to CNN.

“In the beginning he was a normal being that cared about family, loved to joke, loved to have fun, but then a few months after we were married I saw his instability. I saw that he was bipolar and he would get mad out of nowhere. That’s when I started worrying about my safety,” she said.

The couple married in 2009 and divorced in 2011, according to St. Lucie County records.

Syed Shafeeq Rahman, imam of the Islamic Center of Fort Pierce, where Mateen worshipped since childhood, said that as a grown-up, Mateen kept his thoughts to himself.

“He was really quiet. He came at the last minute and did not talk to anybody,” the imam said. “He would shake hands … and take his son on his shoulders and leave.”

Mateen was born in Queens, N.Y., to parents who are from Afghanistan. They moved to Florida — specifically Port St. Lucie — in 1991, when he was 4 or 5, according to court records.

He attended public schools including Southport Middle School and St. Lucie West Centennial High School but graduated from Martin County Adult Vocational School, records show.

He then enrolled at Indian River Community College, where he received an associate’s degree in criminal justice in 2006, according to school spokesman Robert Lane.

That’s a degree required to become a law enforcement officer in Florida.

The school would not release his transcript.

Mateen was injured in 2005 when a St. Lucie County Sheriff’s deputy lost control of his cruiser while racing to the scene of an automobile crash, spun out of control and hit a tree, according to a crash report.

Mateen, then 19, was a passenger in the patrol car, taking part in a citizen ride-along, according to Sheriff’s Office spokesman Bryan Beaty.

Mateen was not seriously injured but was taken to the hospital in an ambulance.

As a teenager, he held a variety of jobs, including a stint at a Publix bakery in Palm City, Chick-fil-A in Jensen Beach and a Gold’s Gym in St. Lucie West, according to paperwork he filled out at age 19 when he legally changed his name from Omar Mir Seddique to Omar Mir Seddique Mateen.

At the time of his death Mateen lived in Fort Pierce, about 120 miles south of Orlando, and had a wife and 3-year-old son.

He died during a shootout with an Orlando police SWAT team as it stormed Pulse, a gay nightclub just south of downtown, where Mateen had killed 49 people and wounded 53 others.

Just before or during the standoff, he had called 911, according to authorities, and claimed allegiance to the Islamic State.

He was armed with a 9 mm semi-automatic pistol and .223-caliber assault rifle, according to federal authorities, and opened fire about 2 a.m. He exchanged gunfire with Orlando police officers outside the club then retreated inside, where he holed up for three hours, trapping at least 30 people.

Mateen bought his weapons at St. Lucie Shooting Center a few days before the massacre. Owner Edward Henson said Monday he “vaguely” remembered Mateen and is cooperating with authorities.

“He’s a nobody,” said Henson. “He’s a customer. He came and purchased his guns, and he left.”

He said Mateen had to go through the same three-day waiting period as all other new gun buyers.

“I have a business,” he said. “I follow the law, I don’t make the law.”

Nothing in Mateen’s background prohibited him from legally buying those guns.

He was not a convicted felon, was not facing a felony charge or a misdemeanor domestic violence charge. He was not a drug abuser, a fugitive, the subject of a domestic violence injunction, someone in the country illegally, someone who’d been dishonorably discharged from a branch of the military or someone who had been found by a judge to be mentally incompetent.

Even though the FBI had twice investigated him, that had no bearing on his ability to buy a gun.

“You cannot be deprived of rights because of an accusation,” said Eileen Rieg, a former gun shop owner in Orlando who now operates a security guard training company.

At a news conference Monday morning, Orlando Police Chief John Mina said during the standoff a team of negotiators talked to Mateen, who he said had retreated to a bathroom with several hostages.

Mina said he ordered the assault on the building at 5 a.m. after it became clear that Mateen intended to harm more people.

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(Orlando Sentinel staff writers Lauren Ritchie, Elyssa Cherney, David Fleshler and David Harris contributed to this report.)

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