What could have driven Omar Mir Seddique Mateen, a New York-born security guard who was the son of Afghan immigrants, to such a horrific act of violence -- the biggest terror attack on U.S. soil since 9/11?
Descriptions have quickly emerged of a troubled and angry individual.
His ex-wife, Sitora Yusufiy, described a brief but violent relationship to a mentally ill man whom she was only able to escape from through her family's help. She said he was physically abusive and a steroid abuser.
And an ex-colleague who worked as a security guard alongside Mateen between 2014 and 2015 said his aggressive behavior eventually drove him to quit his job.
"He was an angry person, violent in nature, and a bigot to almost every class of person," Dan Gilroy told CNN affiliate WPTV-TV in West Palm Beach. The former police officer asserts that he foresaw Mateen eventually committing an act of mass violence.
Mateen had even come to the attention of authorities, with the FBI interviewing him in two terror-related cases in recent years.
But both of the investigations were closed, and Mateen -- who would go on to call 911 and pledge allegiance to ISIS during his rampage -- was not under investigation or surveillance at the time of the attack.
Despite the red flags Mateen raised in his interactions with others, the mass killer was able to purchase a handgun and assault rifle legally in the days before the massacre, Trevor Velino of the Bureau of Alcohol, Tobacco, Firearms and Explosives told reporters.
He had tried to buy body armor, but the store where he tried to make the purchase doesn't sell that product, according to a store manager.
Cell phone tower data indicated he spent several hours Saturday at Disney Springs -- the shopping and entertainment complex inside the Walt Disney World Resort -- just prior to the attack, law enforcement officials said.
Mateen is believed to have been alone during that period, they said.
Four regular patrons at the Pulse nightclub, where the massacre took place, told the Orlando Sentinel they had seen Omar Mateen there before. In interviews with the newspaper, they said that he'd been there multiple times.
Prior visits to Pulse are a line of inquiry investigators are pursuing, sources involved in the investigation told CNN's Jim Scuitto and Evan Perez.
FBI had investigated him twice
Mateen first came on the FBI's radar in 2013 when he made "inflammatory comments to co-workers alleging possible terrorist ties," Assistant Special Agent in Charge Ronald Hopper said. But investigators "were unable to verify the substance of his comments," he said.
In 2014, the FBI interviewed Mateen again over possible connections with Moner Mohammad Abu-Salha, a Florida man who became the first known American suicide bomber in Syria. The men frequented the same mosque.
"We determined that contact was minimal and did not constitute a substantive relationship or threat at that time," Hopper said.
FBI Director James Comey said that the agency is "highly confident" Mateen was radicalized, at least in part, by viewing extremism on the internet.
According to one official, analysis of Mateen's electronic devices showed searches for jihadist propaganda, including videos of ISIS beheading.
"He consumed a hell of a lot of jihadist propaganda," the source said.
"There are strong indications of radicalization by this killer and of potential inspiration by foreign terrorist organizations," Comey said.
He said that investigators have found no indication the attack was directed from outside the United States or that Mateen was part of any kind of network.
A report Monday on the official online ISIS radio channel, Al-Bayan, described the attack as a "raid on a Crusader gathering" carried out by "one of the Caliphate's soldiers in America."
But there was no claim the attack was directed, just an after-the-fact assertion the gunman was an ISIS fighter.
Married with a child
Mateen lived in a condo in Fort Pierce, Florida, with his second wife, a woman named Noor Salman, according to documents CNN obtained. He also had a son, 3½, according to Mateen's father.
He had worked for nine years as a security officer at G4S Secure Solutions, one of the world's largest private security companies.
According to a neighbor, he was a security guard at the St. Lucie County Courthouse, often manning the metal detectors at the front of the building.
Ex-wife: He abused me
His first wife, Yusufiy, painted a damning portrait of the killer, describing a physically abusive marriage to a man with anger issues.
Yusufiy, who is originally from Uzbekistan, said the relationship had started well initially after they met online about seven years ago.
"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," she said.
"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 abuse became a regular occurrence.
"He started abusing me physically, very often, and not allowing me to speak to my family, keeping me hostage from them," Yusufiy said.
"(My family) had to pull me out of his arms and find an emergency flight. ... I made a police report."
While her ex-husband was religious, she said, she did not believe his religion played a role in the attack, she said.
Father baffled by killings
Mateen's father, Seddique Mateen, of Port St. Lucie said he was "really puzzled" by his son's actions.
"My family is completely shocked. They can't believe what did he do," he said at a news conference.
"I am very mad," the father said, adding that he would have called authorities if he had a hint that his son planned something like this.
"In the United States I gave him the best education possible," he told WOFL-TV in Orlando.
"We provided for him love and care. The best possible way a father and a mother can provide. So what had happened, it's really surprised me."
The killer was known to worship at the Fort Pierce Islamic Center.
But his father -- who had an occasional television show on an Afghan satellite channel in which he regularly criticized Afghanistan's government and Pakistan -- said he saw no religious motivation in the killing.
"Radicalism? No. He doesn't have a beard even. When someone becomes radical, they grow long beards and they wear clothes that you know, long clothes, and I don't think religion or Islam had nothing to do with this," he said.
He may have pledged allegiance to ISIS because "he wanted to boost himself," he said.
However, he acknowledged an incident where his son expressed outrage at a gay couple displaying affection in Miami.
"A couple, they were touching each other in front of the kids and in front of the public. And that, he was surprised about that," he told WOFL.
Gilroy, Mateen's former co-worker at PGA Village in Port St. Lucie, said Mateen often made homophobic, sexist and racist remarks.
"He would hit things and as he was hitting things, he would yell, and of course there was always curse words involved, and this wasn't seldom, this was all the time."
e said he asked his employers not to be assigned to work alongside Mateen, but this request was denied. At that point, Gilroy told Mateen he didn't want to continue their relationship on a personal level, according to WPTV.
"He acted very negatively toward that. He then started to text me 20 to 30 times a day. Call me 15 to 20 times," he said.
He said he wished he could have done something to prevent the tragedy.
"I saw it coming. I mean everything," he said. "He said he was going to kill a whole bunch of people."