Officials said they were making significant progress in another area - identifying and notifying victims. Police said 49 people were killed in the shooting - not including Mateen, who had been included in the toll released Sunday - and that they had determined the identities of 48 of these victims. They had notified relatives of half of the victims by Monday morning, leaving many others still waiting for word on whether their loved ones were among the injured or dead.
All the bodies were removed from the club by 11 p.m. Sunday, authorities said.
As the investigation into Mateen moved into its second day, many questions remained unanswered - including what, specifically, might have motivated him. Bentley said investigators were serving search warrants, and the FBI asked anyone with information about the 29-year-old's life to call 1-800-CALLFBI.
During his 911 call, Mateen referenced the 2013 bombing of the Boston Marathon, according to federal law enforcement officials, who spoke on the condition of anonymity because the FBI investigation is unfolding.
While Mateen discussed the Islamic State during a 911 call with police, the level of possible ties between the gunman and the militant group were unclear.
The Islamic State's al-Bayan Radio described him Monday as "one of the soldiers" of its self-described caliphate, but it offered no further details on possible contact before the attack, said the SITE Intelligence Group, which monitors statements by extremist factions. If it does appear the Orlando gunman was radicalized from material available online, it would follow a pattern seen in earlier shooting rampages in San Bernardino, California, and Chattanooga, Tennessee, last year.
A U.S. official, speaking to reporters on condition of anonymity to discuss intelligence assessments, said that attacks inspired by the Islamic State, even when conducted without support from the group's core operation, helped illustrate to followers that they remained a significant military force despite loss of territory in Iraq and Syria over the last year.
"In a sense, inspired attacks and attacks conducted in their name globally" allows them to perpetuate a key perception: continued expansion, the official said. Such attacks show would-be supporters that "they're very much still alive and potent."
Orlando Police Chief John Mina said during a briefing Monday that Mateen did not make many demands during the 911 call discussing the Islamic State, but that his reference to explosives prompted police to head inside Pulse.
Authorities went inside after using an explosive and then an armored Bearcat to break a hole in the wall, Mina said. Hostages came out through a hole in the building, and Mateen - armed with a pair of guns - came out as well. During the gun battle, Mateen was killed and one Orlando police officer was injured when a bullet struck his Kevlar helmet.
However, much still remains unclear, including whether any hostages might have been injured or killed by crossfire.
Police first encountered Mateen shortly after the initial gunfire at about 2 a.m., Mina said. An off-duty officer in uniform was working at the club and engaged Mateen in a gun battle "near one of the entrances." Additional officers called to the scene soon joined in another gun battle, at which point Mateen retreated further into the building and, eventually, into a bathroom.
The gunfire stopped for about three hours, police said. While they spoke with Mateen on the 911 call, no additional shots were heard, and Mina said the gunman was "cool and calm." Mateen not demand much, Mina said. "We were doing most of the asking," the police chief added.
Mateen had been in a bathroom with four or five people, while another 15 or 20 were in another bathroom, Mina said. In his news conference, Mina acknowledged that it was possible hostages were struck in the crossfire: eight or nine SWAT officers, he said, were firing against the backdrop of a concrete wall. But going inside when they worried more people could die "was the right decision to make," Mina said.
Meanwhile, investigators continued to hunt for clues about the lone gunman and what pushed him to carry out the early Sunday slaughter.
What is already known has left a host of questions, including the scope of two different FBI probes that looked into Mateen in recent years.
Officials in Afghanistan - the homeland of Mateen's parents - also opened investigations into any possible connections between Mateen and militant groups.
Yet Mateen's father insisted his son had no Islamist terrorism ties and showed no warning signs the day before the shootings that claimed at least 49 lives and left 53 people injured.
Even as authorities piece together the timeline inside the club, the violence resonates far beyond the FBI files and the police tape encircling Pulse. Earlier accounts from police and other authorities had placed the death toll at 50, but authorities clarified Monday that that tally had included Mateen.
Orlando now joins the mournful list of terrorism-linked bloodshed - Brussels in March, Paris and San Bernardino last year, the Boston Marathon in 2013, London in 2005 and other sites - and is certain to strike deep into American debates over gun rights and how far authorities can go to track potential terrorism threats. And the shooting struck a popular gay club on its Latin night.
It also again sharpens focus on the possible role of Islamist recruitment and influence on social media, and the huge challenges for security forces anywhere to confront lone attackers or tightly organized cells.
In an instant, meanwhile, the U.S. presidential race was transformed. The presumptive GOP nominee, Donald Trump, planned Monday to address the shooting in a speech in New Hampshire. Trump has already proposed harsh measures to confront terrorism fears, such as barring foreign Muslims from entering the country. The Orlando shooter was born in New York to parents from Afghanistan.
On Monday morning, Trump appeared to repeatedly accuse Obama of being somehow complicit in the mass shooting or sympathetic with radicalized Muslims. "We're led by a man that either is not tough, not smart, or he's got something else in mind," Trump said.
His expected Democratic rival, Hillary Clinton, urged greater gun control in a Twitter message Sunday and reached out to gays and lesbians, a possibly important bloc in the election.
"To the LGBT community: please know that you have millions of allies across our country. I am one of them," she said.
As details emerged about the victims - a pharmacy worker, a brand manager for a gay-oriented travel agency - so did chilling accounts of the scene inside the club: pulsing music, gunfire, people falling lifeless to the dance floor or bleeding from gaping wounds from Mateen's assault rifle, a variant of the U.S. military's M-16.
Mateen had legally purchased the two guns - which the Bureau of Alcohol, Tobacco, Firearms and Explosives said were an AR-15-type weapon and a 9mm semiautomatic pistol - within "the last few days," according to Trevor Velinor of the ATF.
Chris Hansen, who had just moved to Florida a couple of months earlier, thought the popping sound was part of the music. "It went with the beat almost," he said.
"I turned the music off and basically everyone was just running out," the club's DJ, Ray Rivera, told the Orlando Sentinel. "It was just complete chaos."
From a bathroom, Eddie Justine sent his mother a series of text messages beginning at 2:06 a.m. Three minutes later, he typed: "He's coming." And then: "I'm gonna die." Justine was confirmed as a victim of the attack by Orlando authorities early Monday.
"As Americans, we are united in grief, in outrage, and in resolve to defend our people," Obama said during a brief speech at the White House, where he said the FBI is investigating the Orlando massacre as an act of terrorism. Until Sunday, the 2007 rampage at Virginia Tech - in which 32 people died - was the country's worst mass shooting.
From around the world, condolences and pledges of support poured in. Vigils and memorials were held from New Zealand to Europe. The Eiffel Tower will be lit in rainbow colors Monday evening.
In Afghanistan, the country's chief executive, Abdullah Abdullah, said the Orlando attack "tells us that terrorism knows no religion, boundary and geography. Terrorism must be eliminated."
The Islamic State repeatedly has summarily executed gay people and released videos showing their gruesome deaths. FBI Special Agent Ron Hopper said the bureau was still working to determine whether sexual orientation was a motive in the Orlando attack. He said investigators have found no indication that Mateen had outside help in planning the attack, nor any sign of other suspects or further threats to the public.
Mateen's ex-wife, Sitora Yusifiy, said he beat her repeatedly during their brief marriage, and she called him unstable.
Mateen's father, however, called his son "very dignified." In a video posted to Facebook shortly after midnight, Seddique Mateen, who lives in Florida, called the shooting "tragic" but said his son was "a good son and an educated son."
He said his son shouldn't have carried out the massacre because "God himself will punish those involved in homosexuality."
"I don't know what caused him to shoot last night," said the father, who has hosted a U.S. -based television show on Afghan affairs and describes himself as an important figure in his homeland.
"No radicalism, no," the father told The Washington Post late Sunday from his home in Port St. Lucie, Florida. "He doesn't have a beard even. ... I don't think religion or Islam had anything to do with this."
Since 2007, Mateen had been an employee at G4S, a British-based security firm whose global contracts have included U.S. federal buildings, said John Kenning, the chief executive officer for North America. Mateen's precise role at the company was not immediately clear, but Kenning said it was cooperating with the FBI investigation.
