49 victims after gunman opens fire in Orlando, Florida nightclub

That's when authorities say Omar Mateen emerged, carrying an assault rifle and spraying the helpless crowd with bullets. Witnesses said he fired relentlessly — 20 rounds, 40, then 50 and more. In such tight quarters, the bullets could hardly miss. He shot at police. He took hostages.

When the gunfire finally stopped, 50 people were dead and dozens more were critically wounded in the deadliest mass shooting in modern U.S. history. Mateen, who law enforcement officials said had pledged allegiance to Islamic State in a 911 call around the time of the attack, died in a gun battle with SWAT team members.

Authorities immediately began investigating whether the assault was an act of terrorism and probing the background of Mateen, a 29-year-old American citizen from Fort Pierce, Florida, who had worked as a security guard. The gunman's father recalled that his son recently got angry when he saw two men kissing in Miami and said that might be related to the assault.

The extremist group did not officially claim responsibility for the attack, but the IS-run Aamaq news agency cited an unnamed source as saying the attack was carried out by an Islamic State fighter.

Even if the attacker supported IS, it was unclear whether the group planned or knew of the attack beforehand.

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 purchased at least two firearms legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.

In a separate incident, an Indiana man armed with three assault rifles and chemicals used to make explosives was arrested Sunday in Southern California and told police he was headed to a Los Angeles-area gay pride parade.

The Orlando shooting started about 2 a.m., with more than 300 people inside the Pulse.

"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."

At 2:09 a.m., Pulse posted on its Facebook page: "Everyone get out of Pulse and keep running."

Club-goer Rob Rick said the shooting started just as "everybody was drinking their last sip."

When he heard shots, Rick dropped to the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area where only workers were allowed. People were then able to escape through the back of the club.

Mateen exchanged gunfire with 14 police officers at the club, and took hostages at one point. In addition to the assault rifle, the shooter also had a handgun and some sort of "suspicious device," the police chief said. About 5 a.m., authorities sent in a SWAT team to rescue the remaining club-goers, Police Chief John Mina said.

At first, officers mistakenly thought the gunman had strapped explosives to the dead after a bomb robot sent back images of a battery part next to a body, Orlando Mayor Buddy Dyer said. That prevented paramedics from going in until authorities determined the battery was something that fell out of an exit sign or a smoke detector, he said.

The robot was sent in after SWAT team members put explosive charges on a wall and an armored vehicle knocked it down in an effort to rescue hostages.

Just before 6 a.m., the Pulse posted an update on its Facebook: "As soon as we have any information, we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love."

Authorities were looking into whether the shooter acted alone, according to Danny Banks, an agent with the Florida Department of Law Enforcement.

"This is an incident, as I see it, that we certainly classify as domestic terror incident," Orange County Sheriff Jerry Demings said.

Mateen's father, Seddique Mir Mateen, told NBC News about his son seeing the men kissing a couple of months ago.

"We are saying we are apologizing for the whole incident," Seddique said. "We are in shock like the whole country."