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While the bodies of Omar Mateen’s victims were removed from Pulse nightclub overnight, investigators worked Monday to determine what motivated the Florida killer responsible for the worst mass shooting in U.S. history — a man the FBI knew was sympathetic to terrorism and had previously investigated.

Mateen, who called 911 to pledge allegiance to ISIS during Sunday’s attack, was possibly planning to carry out the massacre at the Orlando gay club for a long time. He attempted to buy military-grade body armor from a local store in recent weeks, according to a U.S. official. Personnel at the store refused to sell him what is known as Level III body armor, which offers more protection than the body armor police typically issue to officers. Mateen had also legally purchased a Glock pistol and a long gun, Trevor Velino of the Bureau of Alcohol, Tobacco, Firearms and Explosives said.

It’s not known if those weapons were used in the attack.

The FBI and local law enforcement are examining Mateen’s employment history. He worked since 2007 as a security officer at G4S Secure Solutions, one of the world’s largest private security companies. The company assigned him to the St. Lucie County Courthouse. CNN’s Evan Perez reported that the security company knew the FBI had interviewed Mateen but he passed the company’s background check.

The FBI interviewed him in 2013 and 2014 after he expressed sympathy for a suicide bomber, Assistant Special Agent Ronald Hopper said. Hopper said the interviews were “inconclusive,” which meant there was no reason to keep them going but did not expound. It remains unclear what, if anything, the interviews covered or why the probes were dropped.

Mateen was not under investigation at the time of Sunday’s shooting and was not under surveillance, Hopper said. That means, Hopper said, Mateen could “legally walk into a gun dealership and acquire and purchase firearms.”

FBI Director James Comey said Monday that the agency is “highly confident” the gunman was radicalized, at least in part, by viewing extremism on the Internet.

Families wait in anguish

Earlier officials had said the dead numbered 50 but clarified that they were including the gunman. Around midday Monday, Florida Attorney General Pam Bondi said the families of at least 36 out of 49 victims have been notified of their loves ones’ deaths.

The city of Orlando is posting names of the deceased on a website after family members have been notified. The youngest person among those who have been named was Luis Omar Ocasio-Capo, 20.

Survivors told horrific stories of the shooting, which began around 2 a.m. Sunday. One person hiding in the bathroom covered herself with bodies for protection. 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.

Mateen initially engaged in a shootout with an officer and then retreated back into the club.

Police tried for hours to negotiate with Mateen, Orlando police Chief John Mina said.

“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.”

People trapped inside the club desperately called and messaged friends and relatives. Some clubgoers said they thought they were going to die.

At around 5 a.m., police used an armored vehicle to punch through a wall and began pulling people to safety. The gunman came out of that hole, too, and exchanged fire with police, who shot him dead, Mina said.

One bartender recalled crouching, horrified, under under the bar.

When police stormed in, they shouted, “If you are alive, raise your hand!”

She did, and was rescued along with others.

At least 53 people suffered wounds in the attack, officials said.

Possible ISIS ties explored

Mateen’s possible ties to ISIS are a big part of the investigation. During the attack, Mateen called 911 to pledge allegiance to the 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.”

People other than Mateen are being investigated in relation to the attack, U.S. Attorney Lee Bentley said.

“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.

Omar Mir Seddique Mateen was born in 1986 in New York. He most recently lived in Fort Pierce, about 120 miles southeast of Orlando. Fearing explosives at the complex where he lived, officials evacuated about 200 people while they looked through his residence for evidence. The FBI search 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, whom CNN interviewed 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 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?”

Scene inside the club

Pulse describes itself as “the hottest gay bar” in the heart of Orlando. The gay, lesbian, bisexual and transgender communities are coming together across the nation to grieve and, many others, from all walks of life, have joined them in solidarity.

On Sunday, President Barack Obama addressed the nation from the White House.

“We know enough to say this was an act of terror and act of hate,” he said. While the violence could have hit any American community, “This is an especially heartbreaking day for our friends who are lesbian, gay, bisexual or transgender.”

It was another time the President had to try to comfort a nation shaken by gun violence. There was Fort Hood, Texas, in 2009, the 2012 Aurora, Colorado movie theater slayings, the shooting at Wisconsin Sikh temple in 2012, the massacre at Sandy Hook Elementary in 2012, last year’s Charleston, South Carolina, church attack and several others. One of the more recent shootings happened on December 2 in San Bernardino, California, when an ISIS-sympathizer and his wife mowed down co-workers.

National media attention was already focused on Orlando because of Friday night’s fatal shooting of singer Christina Grimmie as she signed autographs after a show. Her shooter then killed himself. The Pulse shooting happened a few miles from the Plaza Live theater, where Grimmie was killed.

‘This is a hate crime’

Numerous groups and officials 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 Pulse nightcub, 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.”