Asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities had "suggestions that individual has leanings towards that."

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."