ORLANDO -- Officials said Monday they have identified all 49 victims killed at a gay nightclub in Orlando in the deadliest mass shooting in U.S. history.
The number of victims was revised down to 49 from 50 as an earlier toll from officials had included the gunman, FBI Special Agent in Charge Paul Wysopal said.
As Americans mourned the worst terror attack on U.S. soil since 9/11, police investigated the gunman's ties to ISIS and examined whether others could have been involved.
People other than Mateen are being investigated in relation to the attack, U.S. Attorney Lee Bentley said, adding that he did not know if anyone else would be charged.
"If anyone else was involved in this crime, they will be prosecuted," he said, adding there was "no reason to believe that anyone connected to this crime" was placing the public in imminent danger.
New details emerge
Officials revealed fresh details Monday about the attack, which began when Mateen, a security guard from Fort Pierce, Florida, entered the packed club at about 2 a.m. Sunday with an assault rifle and a pistol and started shooting, leaving 49 dead and at least 53 wounded.
During the attack, Mateen called 911 to pledge allegiance to the ISIS terror group and mentioned the Boston Marathon bombers, according to a U.S. official.
ISIS sympathizers have reacted by praising the attack on pro-ISIS forums, while the official online ISIS radio channel, Al-Bayan, described it as a "raid on a Crusader gathering" carried out by "one of the Caliphate's soldiers in America."
Orlando police Chief John Mina told reporters Monday that police crisis negotiators tried to negotiate with Mateen during the attack, to no avail.
"He was cool and calm when he was making those phone calls to us," Mina said. "We had a team of crisis negotiators that did talk to the suspect just trying to get as much information as possible. ... He really wasn't asking for a whole lot. We were doing most of the asking."
After a standoff of about three hours, while people trapped inside the club desperately called and messaged friends and relatives, police crashed into the building with an armored vehicle and stun grenades, officials said.
The gunbattle that killed Mateen began after he emerged through a hole that a police armored vehicle had made in the building in an attempt to rescue hostages, Mina said.
"The suspect came out of that hole himself ... and engaged in a gunbattle with officers, where he was ultimately killed," Mina said.
'An act of hate'
"We know enough to say this was an act of terror and act of hate," President Barack Obama said in an address Sunday to the nation from the White House.
While the violence could have hit any American community, he said, "This is an especially heartbreaking day for our friends who are lesbian, gay, bisexual or transgender."
Omar Mir Seddique Mateen was born in 1986 in New York. Most recently he lived in Fort Pierce, about 120 miles southeast of Orlando. Fearing explosives at the apartment complex where he lived, officials evacuated about 200 people while they looked through his residence for evidence. The FBI search of the apartment has now been completed, a law enforcement source told CNN.
Mateen's parents, who are from Afghanistan, said he'd expressed outrage after seeing two men kiss in Miami, but they didn't consider him particularly religious and didn't know of any connection he had to ISIS.
He was married in 2009 to a woman originally from Uzbekistan, according to the marriage license, but he filed documents to end the marriage in 2011.
Sitora Yusufiy, interviewed by CNN in Boulder, Colorado, said she and Mateen were together about four months, though it took a long time to complete the divorce because they lived in different parts of the country after separating.
Mateen was a normal husband at the beginning of their marriage but started abusing her after a few months, she said. She said Mateen was bipolar, although he was not formally diagnosed. She also said he had a history with steroids. He was religious but she said she doesn't think his religion played a role in the attack.
According to a mortgage document, Mateen remarried a woman named Noor Salman and they lived in a Fort Pierce condo. CNN's Sara Sidner spoke to a source in Rodeo, California, who knows the family well. The source said Salman's mother wept and tearfully asked: "Why is God doing this to me?"
Mateen had worked since 2007 as a security officer at G4S Secure Solutions, one of the world's largest private security companies.
FBI Assistant Special Agent Ronald Hopper said the agency was aware of Mateen. The FBI interviewed him in 2013 and 2014 after he expressed sympathy for a suicide bomber, Hopper said.
"Those interviews turned out to be inconclusive, so there was nothing to keep the investigation going," Hopper said.
Mateen was not under investigation at the time of Sunday's shooting and was not under surveillance, Hopper said.
In the past two weeks, Mateen legally purchased a Glock pistol and a long gun, Trevor Velino of the Bureau of Alcohol, Tobacco, Firearms and Explosives told reporters.
It's not known if those weapons were used in the attack.
"He is not a prohibited person. They can legally walk into a gun dealership and acquire and purchase firearms. He did so. And he did so within the last week or so," Velinor said.
Scene inside the club
Pulse describes itself as "the hottest gay bar" in the heart of Orlando. Hours before the shooting, the club urged partygoers to attend its Latin event Saturday night. The club is a vast, open space that was hosting more than 300 patrons late Saturday and into Sunday morning.
People inside the nightclub described a scene of panic made more confusing by the loud music and darkness.
"At first it sounded like it was part of the show because there was an event going on and we were all having a good time," clubgoer Andy Moss said. "But once people started screaming and shots just keep ringing out, you know that it's not a show any more."
Christopher Hansen said he was getting a drink at the bar about 2 a.m. when he "just saw bodies going down." He heard gunshots, "just one after another after another."
The gunshots went on for so long that the shooting "could have lasted a whole song," he said.
When the shots began, Hansen hit the ground, crawling on his elbows and knees, before he spotted a man who had been shot.
"I took my bandana off and shoved it in the hole in his back," Hansen said, adding that he saw another woman who appeared to be shot in the arm.
Survivors provided CNN with dramatic accounts of how they avoided death. One person hiding in the bathroom covered herself with bodies to protect herself. Some entertainers hid in a dressing room when the shooting started and escaped the building by crawling out when police removed the air conditioning unit.
One of the bartenders said she hid under the glass bar. Police came in and said, "If you are alive, raise your hand." Then police got her and others out.
After the initial shooting, police surrounded the club while Mateen was inside, as clubgoers hid in bathrooms and other parts of the building. People inside the club were communicating on their phones with law enforcement from that time until around 5 a.m., when authorities used an armored vehicle to break down the door of the building.
Hospital swamped with victims
The city of Orlando is posting names of the deceased on a website after next of kin are notified. The youngest person among those who have been named was Luis Omar Ocasio-Capo, was 20.
Before Sunday, the deadliest shootings in U.S. history were at Virginia Tech in 2007 and Sandy Hook Elementary School in 2012, with 32 and 27 killed. Fourteen people were killed December 2 in an attack in San Bernardino, California.
National media attention was already focused on Orlando over the weekend because of Friday night's fatal shooting of Christina Grimmie while she signed autographs after a show. She was an up-and-coming singer who had appeared on NBC's "The Voice." Her shooter then killed himself. The Pulse shooting happened only a few miles from the Plaza Live theater, where Grimmie was killed.
Dyer, the mayor, called for the city to come together. "We need to support each other. We need to love each other. And we will not be defined by a hateful shooter," he said.
Obama called for flags to be lowered to half-staff and Florida Gov. Rick Scott called for a moment of silence across the nation at 6 p.m. Sunday. States of emergency were declared for the city of Orlando and for Orange County.
Santa Monica arrest
Also Sunday, authorities in West Hollywood, California, arrested James Wesley Howell, 20. They said they found an arsenal in his car that included, three assault rifles, high-capacity magazines ammunition and a five-gallon bucket with chemicals that could be used to create an explosive device.
There was no indication the arrest was related to the Orlando attack.
'This is a hate crime'
Numerous groups denounced the attacks, including the Vatican, Afghan President Mohammad Ashraf Ghani and the Council on American-Islamic Relations.
"This is a hate crime, plain and simple," CAIR National Communications Director Ibrahim Hooper said. "We condemn it in the strongest possible terms."
Nadine Smith of Equality Florida said groups are planning vigils around the nation. "It has just been a devastating day but people are starting to rally," she said.
Barbara Poma, owner of the Orlando nightclub Pulse, said her employees had been like family to her.
"From the beginning, Pulse has served as a place of love and acceptance for the LGBTQ community," she said. "I want to express my profound sadness and condolences to all who have lost loved ones. Please know that my grief and heart are with you."