FORT PIERCE: Like many other young American men, Omar Mateen worked a series of unremarkable jobs during and after high school ─ a Publix grocery store, Circuit City, Chick-Fil-A and Walgreens.

He graduated in 2003 and jumped through a series of jobs related to nutrition and health, working at Nutrition World, Gold's Gym and a GNC store.

He worked for six months at a state prison. And an early marriage faltered.

But eventually he found some form of stability with a job as a security guard in South Florida, where he was still working when he stormed a gay nightclub early Sunday in Orlando, resulting in the deaths of 49 people and the wounding of 53 more. He died in a gunfight with police.

It remained unclear exactly why Mateen targeted the club. However, one patron told The Orlando Sentinel he saw Mateen drinking at the bar several times before the shooting. Ty Smith said he saw Mateen inside at least a dozen times.

"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," Smith said.

"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."

Kevin West, who also frequented the club, told the Los Angeles Times that Mateen messaged him occasionally using a gay chat and dating app, but the two never met or knew each other by name.

West saw Mateen enter the club an hour before the shooting began and turned over his cellphone and log-in information to the FBI after the shooting.

Mateen, 29, spent most of his childhood on Florida's Atlantic coast and lived there as an adult, not far from his parents.

Born in the New York City borough of Queens in 1986, he moved with his family to the Long Island town of Westbury two years later and then in 1991 to Port Saint Lucie, Florida, about 125 miles southeast of Orlando.

Imam Syed Shafeeq Rahman of the city's Islamic Center said he knew Mateen and his family since the shooter was a boy and never saw any signs of violence in him. The Orlando attack, he said, "was totally unexpected".

But one colleague at the security contractor G4S, his most recent employer, remembered the son of Afghan immigrants as an angry, profane co-worker who used slurs and threatened violence.

Daniel Gilroy said Mateen started badgering him and sending dozens of text messages to him daily, and that he reported Mateen's behavior to his bosses.

“I kind of feel a little guilty that I didn't fight harder,” Gilroy said. "If I didn't walk away and I fought, then maybe 50 people would still be alive today."

Gilroy told multiple news outlets that Mateen routinely used slurs for gay people, blacks, Jews and women. "He talked about killing people all the time," Gilroy told The New York Times.

Of the massacre, Gilroy said, "I saw it coming."

G4S denied Gilroy's claims, saying in a statement Monday that it had no record of Gilroy ever making such a complaint to his superiors.

Furthermore, the company said, Gilroy told officials shortly after leaving in June 2015: "The work and job assignments were respectful and co-workers were good men and women that put in an honest day's work and genuinely like to work as a team and contribute."

A woman who said she was once a neighbor of Mateen's said the security company assigned him to guard the back entrance of PGA Village, a golf resort in Port St. Lucie. Tricia Adorno said her children would sometimes play ball with Mateen's young son.

A spokesman for the St. Lucie County clerk of court said Mateen had once been assigned to the clerk's building and was issued a credential. Joseph Abreu said Mateen last used the credential to access the building in May 2013.

In a statement, G4S said Mateen had been subject to a detailed screening when he was recruited in 2007 and was re-screened in 2013 "with no adverse findings".

Mateen graduated with a degree in criminal-justice technology from Indian River Community College in 2006, the same year he changed his name from Omar Mir Seddique to Omar Mir Seddique Mateen, according to civil court records in St. Lucie County.

The papers listed no reason for the name change.

His ex-wife, Sitora Yusufiy, said her former husband wanted to be a police officer and had applied to the police academy. Florida Department of Corrections spokesman Alberto Moscoso said Mateen had worked at the Martin Correctional Institution in Indiantown, Florida, from Oct 27, 2006, to April 27, 2007.

In September 2007, he was hired by G4S, a global security firm that has 610,000 employees in about 100 countries. Yusufiy told reporters in Boulder, Colorado, that she believed Mateen suffered from mental illness.

Although records show the couple didn't divorce for two years after their 2009 marriage, Yusiufiy said she was actually with Mateen for only four months because he was abusive. She said he would not let her speak to her family and that family members had to come literally pull her out of his arms.

Divorce papers reveal that Mateen made $1,600 a month working for G4S and that she earned $3,000 a month as a real estate agent. It's unclear when Mateen married his second wife, Noor Salman, but an Aug 30, 2013, property deed in Saint Lucie County identified them as a married couple.

Family members said the pair had a young son about 3 years old.

On Monday, the FBI said Mateen appeared to be a "homegrown extremist" who espoused support for a jumble of often-conflicting Islamic radical groups. During the attack, he called 911 to profess allegiance to the militant Islamic State group.

FBI Director James Comey said Mateen was clearly "radicalised," at least in part via the Internet.

The FBI became aware of him in 2013 when co-workers reported that he claimed to have family connections to Al Qaeda and to be a member of Hezbollah, too, Comey said.

The agency launched a 10-month preliminary investigation, following Mateen, reviewing his communications and questioning him, the FBI chief said.

Mateen claimed he made the remarks in anger because co-workers were teasing him and discriminating against him as a Muslim. The FBI eventually closed the case, Comey said.

His name surfaced again as part of another investigation into a suicide bomber from the Syrian rebel group Nusra Front. The FBI found Mateen and the man had attended the same mosque and knew each other casually, but the investigation turned up "no ties of any consequence," Comey said.

Mateen purchased at least two firearms legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.

He attended evening prayer services at Port St. Lucie's Islamic Center three to four times a week, most recently with his son, Rahman said. The imam said he last saw Mateen on Friday.

“When he finished prayer, he would just leave,” Rahman told The Associated Press. "He would not socialise with anybody. He would be quiet. He would be very peaceful."

On Monday, the shooter's father, Seddique Mir Mateen, said what his son did "was the act of a terrorist".

"I apologise for what my son did. I am as sad and mad as you guys are," he told reporters outside his home. He did not go into details about his son's religious or political views, insisting he did not know.

Asked whether he missed his son, the father said: "I don't miss anything about him. What he did was against humanity".