Omar Mateen, a 29-year-old U.S.-born citizen, has been identified as the suspect in Sunday's mass shooting that left at least 50 dead and more than 50 others wounded at the Pulse Orlando, a gay nightclub in Orlando, Florida.
In the midst of the attack, Mateen called 911 and pledged his allegiance to ISIS, according to two law enforcement officials.
During a news conference, FBI officials said Mateen has been under investigation twice in the past.
In 2013, he allegedly made comments to coworkers about possible terrorist ties. The FBI investigated, interviewed Mateen twice and decided to close the case.
The FBI interviewed Mateen in 2014 in connection to a known terrorist -- a suicide bomber.
An ATF official said Mateen was able to acquire a gun permit and purchased two firearms legally. he was not under investigation at this time, according to the report. The ATF official said Mateen purchased at least two firearms within the past week or so.
The ATF later tweeted the guns traced to the shooting:
Sunday afternoon, a live statement from Florida Sen. Bill Nelson indicated that the ISIS news network issued a statement claiming responsibility for the attack. This report was not confirmed by investigators.
Orlando massacre: Read up-to-the-minute coverage, background
Terrorism observers told ABC News that while ISIS supporters have cheered the massacre online, so far there has been no claim of responsibility from any terrorist organization and no evidence of inside knowledge of the plot. ISIS, which is based in Syria, has targeted gay men for beatings and murder in the Middle East and has filmed its members throwing some men suspected of being gay from the roofs of buildings.
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.
Mateen's ex-wife said his family was from Afghanistan but that her ex-husband was born in New York. His family later moved to Florida.
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 shooter in 2013 made inflammatory comments to co-workers, and Mateen was interviewed twice, FBI agent Ronald Hopper said. He called those interviews inconclusive.
In 2014, Hopper said, officials found that Mateen had ties to an American suicide bomber. He 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.
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.
The Orlando suspect exchanged gunfire with 14 police officers at the club, which had more than 300 people inside.
The gunfire started around 2 a.m.
"He had an automatic rifle, so nobody stood a chance," said Jackie Smith, who had two friends next to her get shot. "I just tried to get out of there."
At one point, the gunman took hostages. Around 5 a.m., authorities sent in a SWAT team to rescue them, Police Chief John Mina said.
Pulse initially posted on its own Facebook page: "Everyone get out of Pulse and keep running." Just before 6 a.m., the club posted an update: "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."
In addition to the assault rifle, the shooter also had some sort of "suspicious device," the police chief said.
Thirty-nine of the dead were killed at the club, and 11 people died at hospitals, the mayor said.
At first, officers mistakenly thought the gunman had strapped explosives to the dead and that the club was booby-trapped. A bomb robot sent back images of a battery part next to a body, 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.
The robot was sent in after SWAT team members put explosive charges on a wall and an armored vehicle knocked the wall down in an effort to rescue hostages.
In the aftermath of the attack, police departments across the country stepped up patrols in neighborhoods frequented by the LGBT community.
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."
When asked if the gunman had a connection to radical Islamic terrorism, Hopper said authorities had "suggestions that individual has leanings towards that."
Mateen's father said the attack had nothing to do with religion, he said.
The gunman 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 118 miles southeast of Orlando.
Relatives and friends, many in tears, gathered outside the hospital to learn the fate of loved ones.
Smith did not know the conditions of her wounded friends. She came out of the hospital and burst into tears.
Christine Leinonen drove to Orlando at 4 a.m. after learning of the shooting from a friend of her 32-year-old son, Christopher Leinonen, who was at Pulse and is missing.
She had not heard from her son and feared the worst.
"These are nonsensical killings of our children," she said, sobbing. "They're killing our babies!"
She said her son's friend Brandon Wolf survived by hiding in a bathroom and running out as the bullets flew.
A woman who was outside the club early Sunday was trying to contact her 30-year-old son, Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: "He's coming."
"The next text said: 'He has us, and he's in here with us,'" Mina Justice said. "That was the last conversation."
A bartender said she initially thought the gunshots were music. But after a second shot, there was a pause, followed by more shots. That's when Tiffany Johnson realized something was wrong.
Johnson said people dropped to the ground and started running out of the club. She ran to a fast-food restaurant across the street and met one of her customers who let her get in his car. They drove away.
Club-goer Rob Rick said the shooting started just as "everybody was drinking their last sip."
He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area where only workers are allowed. People were then able to escape through the back of the club.
Christopher Hansen said he was in the VIP lounge when he heard gunshots. He continued to hear shooting even after he emerged and saw the wounded being tended across the street.
"So I just dropped down. I just said, 'Please, please, please, I want to make it out,'" he said. "And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot."
The attack follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, a YouTube sensation and former contestant on "The Voice." She was killed after an Orlando concert by a 27-year-old man who later killed himself.