A massacre at an Orlando gay nightclub early Sunday morning has been described as a "domestic terror incident" with at least 49 dead and 53 injured, officials said, making it the worst mass shooting in U.S. history and the deadliest terror attack on U.S. soil since the events of Sept. 11, 2001.

The shooter has been identified by officials as Omar Mateen of St. Lucie County, Florida, an American-born citizen with Afghani parents. After the shooting began, he called 911 to pledge his allegiance to ISIS, according to law enforcement officials.

The previous deadliest mass shooting in the U.S. was the 2007 attack at Virginia Tech, where a student killed 32 people before killing himself.

Mateen's family was from Afghanistan, and he was born in New York. His family later moved to Florida, authorities said.

His ex-wife, Sitora Yusufiy, told reporters that her former husband was bipolar and "mentally unstable."

Mateen was short-tempered and had a history with steroids, she said in remarks televised from Boulder, Colorado. She described him as religious but not radical. He wanted to be a police officer and applied to a police academy, but she had no details.

The couple was together for only four months, and the two had no contact for the last seven or eight years, she said.

A law enforcement official said the gunman made a 911 call from the club in which he professed allegiance to the leader of the Islamic State, Abu Bakr al-Baghdadi. The official was familiar with the investigation, but was not authorized to discuss the matter publicly and spoke on condition of anonymity.

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.

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, Mir Seddique, 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."

Mateen was a security guard with a company called G4S. In a 2012 newsletter, the firm identified him as working in West Palm Beach. In a statement sent Sunday to the Palm Beach Post, the company confirmed that he had been an employee since September 2007. State records show that Mateen had held a firearms license since at least 2011.

President Barack Obama called the shooting an "act of terror" and an "act of hate" targeting a place of "solidarity and empowerment" for gays and lesbians. He urged Americans to decide whether this is the kind of "country we want to be."

Authorities said they had secured a van owned by the suspect outside the club. Meanwhile, a SWAT truck and a bomb-disposal unit were on the scene of an address associated with Mateen in Fort Pierce, about 120 miles southeast of Orlando.

Across the country, police departments stepped up patrols in neighborhoods frequented by the LGBT community.