Orlando gunman Omar Mateen 'cool and calm' during negotiations

The gunman who killed dozens and took hostages at an Orlando nightclub was "cool and calm" in a phone call with police negotiators, officials revealed Monday.

Fifty people — including the gunman — died in the attack on Orlando's Pulse nightclub early Sunday. More than 50 others were wounded.

Amid an ongoing investigation into a possible motive, Orlando Police Chief John Mina gave more details Monday on the run-up to the SWAT raid that ended the crisis.

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He told an early-morning press conference that gunman Omar Mateen had holed up in a bathroom with hostages after an initial shootout with police.

Orlando Shooting Suspect: Omar Mateen Graphiq

Mateen was "cool and calm" in a conversation with negotiators, according to Mina.

"He really wasn't asking for a whole lot," Mina said. "We were doing most of the asking."

There was talk of "bombs and explosives" — plus a reference to ISIS, according to Mina.

He said that once information from hostages and the suspect suggested "further loss of life was imminent," a decision was taken for SWAT to breach the wall of the club.

Mateen was armed with an assault-style weapon and a pistol. A third weapon was found in his car, authorities said Monday.

Around 100 leads have emerged in the ongoing investigation.

"No stone will be left unturned and we'll follow the leads wherever they take us," FBI special agent Paul Wyposal told Monday's press conference.

U.S. Attorney Lee Bentley III said there was "no reason to believe" anyone connected to the crime posed an "imminent danger" to the public.

Teams of agents have been working "around the clock" to process a "great amount" of electronic and physical evidence, he added, saying the investigation was "still in the early stages."

Mateen first came to the FBI's attention in 2013 after co-workers reported he'd made "inflammatory" comments to them about radical Islamic propaganda.

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A year later the FBI looked into him again because of his ties to an American who traveled to the Middle East to become a suicide bomber. Agents determined contact there was minimal and Mateen was not under surveillance on Sunday, when he carried out the deadliest mass shooting in U.S. history.

Law enforcement sources earlier told NBC News that Mateen swore allegiance to the leader of ISIS in a 911 call moments before the rampage at Pulse.

However, law enforcement officials have said there was not yet any indication Mateen was in touch with terrorists overseas or that the attack was directed by anyone else.

As the investigation continued officials were working to notify victims' next-of-kin. Orlando Mayor Buddy Dyer said 48 of the 49 victims had been identified and 24 next of kin had been contacted.

He praised the heroics of responding officers and medical personnel, saying that "hundreds of lives were saved."

"We will not be defined by the act of a cowardly hater," Dyer told a press conference. "We will be defined by how we respond.

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Florida Gov. Rick Scott confirmed he had asked President Barack Obama to declare a state of emergency in wake of the shooting.

"Right now it's time to grieve," he told a press conference.