The gunman who opened fire inside a crowded gay nightclub in Orlando, Florida, early Sunday, killing 49 and wounding 53, 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, identified by authorities as Omar Mateen, a 29-year-old Florida resident who was born in New York City, 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 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."
With the investigation still in the early stages, indications are that Mateen wanted to be associated with the terrorist group ISIS and was upset by seeing a gay couple kiss, according to an NBC News investigation. But it wasn't clear what drove him to carry out the rampage at Pulse nightclub, the deadliest shooting in the United States. Mateen died in a shootout with SWAT team.
Mateen's ex-wife, Sitora Yusufiy, said in a news conference in Boulder, Colorado, Sunday, Mateen wanted to be a police officer and had a license to own a gun in Florida.
"There was no sign of any of this at all," said Yusufiy.
Yusufiy's fiance, Macio Dias, said mental illness is to blame for Mateen's actions.
"No one ever expected that he would do it. He was a man who was very confused and troubled," Dias said.
Mateen worked as a security guard in Florida. Daniel Gilroy, a co-worker at G4S Secure Solutions, told NBC News Mateen often showed up early for work and was fascinated by law enforcement. Gilroy also describred the gunman as "very excitable" and racist.
"He was scary in a concerning way," Gilroy said. "And it wasn't at times. It was all the time. He had anger management issues. Something would set him off, but the things that would set him off were always women, race or religion. [Those were] his button pushers."
Gilroy said he asked for a transfer out of the "toxic" work environment.
Mateen, who lived in Fort Pierce, Florida, made inflammatory comments in 2013 indicating he had possible terrorist ties, but they could not be substantiated, according to FBI official Ron Hopper.
He was investigated again in 2014, due to possible ties to an American suicide bomber, but a connection could not be substantiated.
The attack came during national LBGT pride month, and an NBC News interview with Mateen's father, Mir Seddique, indicated that a hatred of gay people may have contributed to the shooting.
Mateen was angered when he saw two men kissing in Miami several months ago, Seddique said. He suggested that might be related to the attack, which he said had nothing to do with religion.
"We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry," Seddique told NBC News on Sunday. "They were kissing each other and touching each other and he said, 'Look at that. In front of my son they are doing that.' And then we were in the men's bathroom and men were kissing each other."
Seddique also said Mateen was a husband and father to a 3-year-old boy. Mateen worked in security and attended Indian River State College, NBC News reported. A spokeswoman said he got an associate of science degree in criminal justice technology in 2006.
A law enforcement official told NBC News Sunday there's no indication so far that Mateen was in touch with terrorists overseas and that it was a directed attack.
Several officials told NBC News there's no sign that anyone else was involved in the attack, either in providing assistance or egging him on.
The Islamic State's radio called Mateen "one of the soldiers of the caliphate in America." Al-Bayan Radio, a media outlet for the IS extremist group, on Monday hailed the attack, saying it targeted a gathering of Christians and gays and that it's the worst attack on U.S. soil since the September 11, 2001, terrorist attacks. ISIS has not officially claimed responsibility for the Orlando attack.
President Barack Obama delivered a somber address to the nation Sunday afternoon, saying of Mateen: "What is clear is that he was a person filled with hatred."
Obama noted his administration has "reached no definitive judgment on the precise motivations of the killer."
Police said he used a handgun and AR-15-type rifle in the rampage and ATF officials tweeted Sunday that he legally purchased the firearms within the last week.
Records show he had filed a petition for a name change in 2006 from Omar Mir Seddique to Omar Mir Seddique Mateen, according to NBC News.
A man who picked up the phone at Mateen's listed address, Mustafa Abasin, told NBC News: "We are in shock and we are sad." He would not explain how he knew the gunman, but added that he was aiding investigators.
Earlier on Sunday, The Washington Post published an interview with a woman who said she was Mateen's ex-wife, who said he beat her repeatedly.
"He was not a stable person," she told the Post. She said his family was originally from Afghanistan and confirmed that photos posted online on Myspace were of Mateen, some in which he wore shirts with the NYPD logo.
She said their marriage lasted only a few months, but during that time she never noticed him becoming radicalized. The couple divorced in 2011.