He was a body builder and a security guard, a religious man who attended the local mosque and wanted to become a police officer.
Early Sunday, 29-year-old Omar Mateen opened fire at a gay nightclub in Orlando. Police said 49 victims died and 53 were wounded. Mateen also was killed.
Mateen called 911 shortly before the massacre to express allegiance to the leader of ISIS, Abu Bakr al-Baghdadi, law enforcement sources told NBC News.
The sources said the gunman also mentioned the brothers who planted bombs at the 2013 Boston Marathon during the calls.
Mateen was "cool and calm" in a conversation with negotiators while holding people hostage and made references to "bombs and explosives" and to ISIS, according to Orlando Police Chief John Mina.
"He really wasn't asking for a whole lot," Mina said at a Monday morning news conference. "We were doing most of the asking."
President Obama said Monday Mateen was inspired by extremist information over the internet and called the attack an apparent example of "homegrown extremism" that U.S. officials have been worrying about for years.
Obama, speaking in the Oval Office after meeting with the FBI director, said the attack appears similar to the shooting late last year in San Bernardino, California, though he added that "we don't yet know." He said the probe was being treated as a terror investigation and that investigators were examining materials from the internet that the shooter may have consumed.
"The fact that it took place at a club frequented by the LGBT community I think is also relevant," Obama said. The attack took place at a gay nightclub.
Mateen was the son of an Afghan immigrant who had a talk show in the United States, the nature of which was not entirely clear: A former Afghan official said the program was pro-Taliban and a former colleague said it was enthusiastically pro-American.
He attended evening prayer services at the city's Islamic Center three to four times a week, most recently with his young son, said Imam Syed Shafeeq Rahman. Although he was not very social, he also showed no signs of violence, Rahman said. He said he last saw Mateen on Friday.
"When he finished prayer he would just leave," Rahman told The Associated Press. "He would not socialize with anybody. He would be quiet. He would be very peaceful."
He was also bipolar, Mateen's ex-wife, Sitora Yusufiy, told reporters in Boulder, Colorado.
"He was mentally unstable and mentally ill," Yusufiy said. Although records show the couple didn't divorce for two years after the marriage, Yusiufiy said she was actually only with Mateen for four months because he was abusive. She said he would not let her speak to her family and that family members had to come and literally pull her out of his arms.
Authorities immediately began investigating whether Sunday's attack was an act of terrorism. A law enforcement official said the gunman made a 911 call from the nightclub professing allegiance to the leader of the Islamic State, Abu Bakr al-Baghdadi. The law enforcement official is familiar with the investigation but was not authorized to discuss the matter publicly and spoke on condition of anonymity.
Federal officials told NBC News Mateen travelled twice to the Middle East -- to Saudi Arabia in 2011 and to the UAE in 2012. The Saudi foreign ministry said he was in Saudi Arabia during those trips to take part in Muslim pilgrimages -- 10 days in 2011 and eight days in 2012. The FBI wants to know more about those trips, according to NBC News.
Two law enforcement sources told NBC News Mateen may have traveled to Disney World to plan an attack there. One source said it was "pre operational," meaning he had not developed a plan. A second source was less certain that he had even gotten that far, and said there is the possibility it was simply a social visit.
Federal officials also also looking into whether Mateen's closest family members, including his wife, had any hints what he was planning and failed to speak up about it.
One law enforcement official said they don't have any conclusions yet, "but it's something we're pursuing right now."
U.S. Attorney Lee Bentley said Monday morning "there is an investigation of other persons" in the the Orlando shooting case.
Yusufiy said she was "devastated, shocked, started shaking and crying" when she heard about the shooting, but she attributed the violence to Mateen's mental illness, not any alliance with terrorist groups.
"My personal opinion is that this has nothing to do with ISIS," he said.
Seddique Mir Mateen, the father of the alleged shooter, is a life insurance salesman who started a group in 2010 called Durand Jirga, Inc., according to Qasim Tarin, a businessman from California who was a Durand Jirga board member. The name refers to the Durand line, the long disputed border between Afghanistan and Pakistan.
Tarin said Seddique Mir Mateen had a television show on which they discussed issues facing Afghanistan.
"It's shocking," he said about the shooting. "(Omar Mateen's) father loves this country."
Some of Seddique Mir Mateen's shows were taped and later posted on YouTube. During one episode, a sign in the background read: "Long live the U.S.A! Long live Afghanistan. ... Afghans are the best friends to the U.S.A."
But a former Afghan official said the "Durand Jirga Show" appears on Payam-e-Afghan, a California-based channel that supports ethnic solidarity with the Afghan Taliban, which are mostly Pashtun. Viewers from Pashtun communities in the United States regularly call in to the channel to espouse support for Pashtun domination of Afghanistan over the nation's minorities, including Hazaras, Tajiks and Uzbeks, the official said.
The "Durand Jirga Show" expresses support for the Taliban, has an anti-Pakistan slant, complains about foreigners in Afghanistan and criticizes U.S. actions there, the official said. Seddique Mir Mateen lavished praise on current Afghan President Ashraf Ghani when he appeared on the show in January 2014, but he has since denounced the Ghani government, according to the official, who said that on Saturday, Seddique Mateen appeared on the show dressed in military fatigues and used his program to criticize the current Afghan government.
He also announced on that show that he would run in the next Afghan presidential election, said the official, who spoke only on condition of anonymity because he did not want to be linked to coverage of the shooting.
In 2013, Omar Mateen made inflammatory comments to co-workers, and he was interviewed twice, FBI agent Ronald Hopper said. He called those interviews inconclusive. In 2014, Hopper said, officials found that Mateen had ties to an American suicide bomber. He described the contact as minimal, saying it did not constitute a threat at the time.
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.
Mateen had no criminal record. Yusufiy said he wanted to be a police officer and had applied to the police academy. Mateen was a security guard at the G4S Secure Solutions, which identifies itself on its website as "the leading global integrated security company."
Mateen changed his name from Omar Mir Seddique to Omar Mir Seddique Mateen in 2006, the same year he graduated from Indian River Community Colleg, according to court documents.
The documents released Monday show he was born in Queens, New York, and moved to Port Saint Lucie in 1991. Between 2001 and 2006, he worked at eight jobs, including a Publix grocery store, Circuit City, Chick-Fil-A and a Walgreens drug store.
He also worked at Nutrition World in Fort Pierce, Gold's Gym and a GNC store in a mall.
Daniel Gilroy told multiple news outlets that he worked with Mateen at G4S. Gilroy called him an angry, loud, profane man who used slurs for gay people, blacks, Jews and women. "He never used other words to describe them," Gilroy told the Tampa Bay Times. Mateen also regularly made threats of violence, according to Gilroy.
"He talked about killing people all the time," Gilroy told The New York Times. And Gilroy wasn't surprised when he learned of the massacre: "I saw it coming."
Gilroy said Mateen started badgering him and sending dozens of text messages to him daily. Gilroy said 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."
But Rahman said Mateen was not, as far as the imam could see, someone who would ever commit such a gruesome act of mass violence. Rahman said he knew Mateen and his family since the shooter was a young boy. Playful as a child, he became more serious as an adult, Rahman said. He spoke both English and Farsi, and was into body building
"It was totally unexpected," Rahman said.