The photo from Omar Mateen's high school yearbook is hardly remarkable - a toothy, dimpled smile with a peach-fuzz mustache below a mop of black hair.

His transformation from high school football player to perpetrator of America's worst mass shooting raises questions about whether red flags were missed over the depth of his apparent sympathies with Muslim extremists.

As families of the victims grieved and the nation recoiled at the scale of yet another mass shooting, a picture began to emerge of the 29-year-old killer as a quiet, devout person who in recent years displayed a hateful and violent streak.

Early on Sunday, he stormed a packed gay nightclub in Orlando, Florida, with a handgun and AR-15 semi-automatic rifle, fatally shooting 50 people before police killed him. Fifty-three others were wounded, many critically.

His ex-wife, Sitora Yusufiy, described him as "bipolar," and emotionally disturbed with a violent temper. She said she had been beaten and otherwise physically abused by Mateen during outbursts in which he would "express hatred towards everything". She was "rescued" by family members just four months into a stormy marriage that began in 2009 and ended in divorce, she said.

"He would often get into fights with his parents, but as I was the only one in his life most of the violence was directed towards me," she told reporters in Boulder, Colorado outside a home where she was staying.

She said he aspired to be a police officer and had worked as a correctional officer at a detention center for juvenile delinquents in Fort Pierce, Florida, and had once sought admission to a police academy.

In Fort Pierce on Florida's southeast coast, 120 miles (195 km) from the shooting, the imam at the mosque that Mateen attended for nearly 10 years described him as a regular worshipper who was quiet and rarely interacted with the congregation.

"He hardly had any friends," Syed Shafeeq Rahman, who heads the Islamic Center of Fort Pierce, told Reuters. "He would come with his little son at night to pray and after he would leave."

Rahman said Mateen never approached him regarding any concerns about homosexuals. He usually prayed at the mosque a few times a week, mostly in evenings and as recently as Friday, but he didn't display signs of radicalism, according to fellow worshippers interviewed by Reuters.

Mateen was born in New York of Afghan descent but spent most of his life in Florida, attending Martin County High School in Stuart, a small city about a 20-minute drive from the Fort Pierce condominium where had most recently lived.

A classmate described him as a typical teen who played football. A school yearbook image of Mateen was seen by Reuters.

Samuel King, who was one year ahead of Mateen, said the two often spoke after Mateen graduated in 2004. King waited tables at Ruby Tuesday's restaurant at Treasure Coast Square, a mall where Mateen worked at GNC, the nutrition store, he said.

King, who is openly gay, said the Mateen he knew until 2009 did not appear to be anti-homosexual.

"What is shocking to me is that the majority of the staff at Ruby Tuesday's when I worked there were gay. He clearly was not anti-(gay) at least not back then. He did not show any hatred to any of us."

While at GNC, Mateen lifted weights and "got really buff," King said, describing Mateen as gregarious and talkative in the immediate years after high school. "Something must have changed" since he last saw him, he added.

Mateen's father, Mir Seddique, told NBC News the massacre was not related to religion. He said his son turned angry when he saw two men kissing in Miami a couple of months ago.

FBI INTERVIEWS

The FBI twice interviewed Mateen for having suspected ties to Islamist militants. The first investigation took place in 2013 when Mateen made inflammatory comments to co-workers that indicated sympathy for militants, FBI special agent in charge Ron Hopper told a news conference in Orlando.

At the time, Mateen worked as a security guard at G4S, a British-owned multinational company that is among the world's largest private security firms.

He joined G4S in September 2007, carried a gun as part of his duties and was employed with the company at the time of the shooting as an armed security officer, the company said.

G4S provides security to federal buildings in Florida.

Mateen was investigated and interviewed twice but the FBI was "unable to verify the substance of his comments," Hopper said.

Daniel Kime, a security guard employed along with Mateen by at G4S in the Fort Pierce area, said he met Mateen briefly three or four times. "Every time I saw him he never smiled. If you said good morning, he'd just walk right by you, like he had a chip on his shoulder," Kime told Reuters.

Daniel Gilroy, who said he was a co-worker of Mateen's at G4S, said that Mateen's anger was "constant."

"Any time a female or a black person would come by he would use horrible words," he told Fox News.

In 2014, Hopper said, Mateen was investigated and interviewed again, this time for suspected connections to Moner Mohammad Abu-Salha, an American citizen who became a suicide bomber in Syria in 2014.

Hopper said Mateen's contact with Abu-Salha was minimal and it was deemed that "he did not constitute a substantive threat at that time."

'NOT A STABLE PERSON'

Hopper said Mateen was not under investigation or surveillance at the time of Sunday's attack.

He said Mateen called during the massacre to pledge allegiance to Islamic State, also known as ISIS, which in recent years declared a caliphate over large swathes of Iraq and Syria. But the depth of that commitment is unclear.

Mateen also mentioned the Boston Marathon bombers during the call, which he made 20 minutes into the shootings, authorities said.

Mateen's former wife said she met Mateen online about eight years ago and decided to move to Florida to marry him, according to the Washington Post.

"He was not a stable person," she said. "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 a Florida firearms license that expired in 2013 and a state permit to work as a security guard, according to public records. He was registered as a Democrat.

City, state and federal officials were searching Mateen's apartment in the Woodlands condominium building in Fort Pierce and had told other residents to evacuate.