The gunman in the Orlando nightclub attack that killed 49 people had "strong indications of radicalization" and was likely inspired by foreign terrorist organizations, FBI Director James Comey said Monday.
The early Sunday attack marked the deadliest mass shooting in modern U.S. history. Shooter Omar Mateen, a 29-year-old American-born Muslim who worked as a security guard in Florida, died in a gun battle with police.
Comey told reporters Mateen spoke with a 911 operator three times early Sunday morning before opening fire on the crowd at Pulse, a gay club that was hosting Latin night. Mateen also pledged loyalty to the head of ISIS on his last call, Comey said.
Mateen was not unknown to law enforcement: In 2013, he made inflammatory comments to co-workers and was interviewed twice, according to FBI agent Ronald Hopper, who called the interviews inconclusive.
In 2014, Hopper said, officials found that Mateen had ties to an American suicide bomber, but the agent described the contact as minimal, saying it did not constitute a threat at the time.
Mateen exchanged gunfire with 14 police officers at the club and took hostages at one point. In addition to the assault rifle, he had a handgun. Police Chief John Mina said officers held back for some time because Mateen indicated he had a bomb vest.
Earlier Monday, President Barack Obama said there was no clear evidence Mateen was directed to conduct his attack or part of a larger plot. He said it appears the shooter was inspired by extremist information disseminated over the Internet.
Obama said the probe is being treated as a terrorism investigation and appears to be similar to last year's shooting spree in San Bernardino, California. Investigators are still looking into the motivations of the shooter, including the fact that the shooting took place at a gay venue.
Two law enforcement sources told NBC News on Monday that Mateen may have traveled to Disney World to plan an attack there. One source said it was "pre-operational," meaning Mateen had not developed a plan. A second source was less certain Mateen had even gotten that far and said it's possible Mateen's visit was simply a social one.
Disney spokesperson Lisa Haines said in a statement, “Unfortunately we’ve all been living in a world of uncertainty, and during this time we have increased our security measures across our properties, adding such visible safeguards as magnetometers, additional canine units, and law enforcement officers on site, as well as less visible systems that employ state-of-the-art security technologies.”
Wielding an AR-15 semi-automatic rifle and a handgun, Mateen opened fire at the crowded club in such close quarters that the bullets could hardly miss. He was gunned down after police used explosives and a small armored vehicle to punch a hole in a wall and allow dozens of clubgoers to escape, police said.
"He had an automatic rifle, so nobody stood a chance," said Jackie Smith, who saw two friends next to her get shot. "I just tried to get out of there."
Mateen bought at least two guns legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.
"I've always felt so safe here for my family, kids. And now, I don't know," said Marlon Massey, who lives across the street from the club, in the city known to tourists around the globe as the home of Walt Disney World and other theme parks.
Obama on Sunday called the shooting an "act of terror" and an "act of hate" against a place of "solidarity and empowerment" for gays.
Mateen's Afghan immigrant father, Seddique Mir Mateen, suggested anti-gay hatred as a possible motive. The father said his son got angry a few months ago when he saw two men kissing in Miami.
Pulse Is More Than a Nightclub, But 'Place of Solidarity': Obama
His father told reporters Monday that the massacre was "the act of a terrorist," and added: "I apologize for what my son did. I am as sad and mad as you guys are."
He wouldn't go into details about any religious or political views his son held, saying he didn't know. Asked whether he missed his son, he said: "I don't miss anything about him. What he did was against humanity."
Mateen's ex-wife attributed the violence to mental illness, saying he was bipolar.
The Islamic State's radio called Mateen "one of the soldiers of the caliphate in America." Al-Bayan Radio, a media outlet for the extremist group, hailed the attack, saying that it targeted a gathering of Christians and gays and that it was the worst attack on U.S. soil since 9/11.
It was unclear, though, whether the group planned or knew of the attack beforehand.
Meanwhile, the last of the bodies were removed from the club overnight as investigators dug into Mateen's background. Workers removed them four at a time on stretchers and loaded them into white vans. The covered bodies were taken to the county medical examiner's office.
'He Was Obviously Disturbed': Omar Mateen's Ex-Wife Speaks Out
"We will not be defined by the act of a cowardly hater," Orlando Mayor Buddy Dyer vowed.
Thirty-nine of the dead were killed at the club, and the others died at hospitals, the mayor said. By Monday morning, families of 24 of the victims had been notified, Dyer said.
At least 53 people were hospitalized, most in critical condition, and a surgeon at Orlando Regional Medical Center said the death toll was likely to climb.
Officials emphasized Monday morning there was no immediate threat to the public.
On the same day as the Orlando attack, an Indiana man armed with three assault rifles and chemicals used to make explosives was arrested in Southern California and told police he was headed to a West Hollywood gay pride parade, authorities said.
The previous deadliest mass shooting in modern U.S. history was the 2007 attack at Virginia Tech, where a student killed 32 people and took his own life.