The American gunman who launched a murderous assault on a gay nightclub in Orlando was radicalized by Islamist propaganda, officials said Monday, as they grappled with the worst terror attack on US soil since 9/11.
The Islamic State group claimed slain shooter Omar Mateen was acting as "one of the soldiers of the caliphate in America" when he attacked the Pulse club in the Florida resort city, an assault that ended when police stormed the venue.
But investigators are probing whether he was a jihadist on a mission or a vicious "lone wolf" inspired by Islamist propaganda to carry out what President Barack Obama dubbed "an act of terror and an act of hate."
Forty-nine people were killed and 53 others wounded in the attack, which was also the deadliest mass shooting in US history.
"We see no indication that this was a plot directed from outside the United States, and we see no indication that he was part of any kind of network," FBI chief James Comey said, adding that investigators were looking into the possible role of anti-gay bigotry.
But he also said the FBI was "highly confident" Mateen had been "radicalized" while consuming online propaganda and that he had claimed allegiance to Islamic State leader Abu Bakr al-Baghdadi in a series of calls during the attack.
The slaughter triggered worldwide shock and outrage, but has also raised questions about US counterterror strategy and gun laws. The suspect was allowed to legally buy a rifle and handgun despite raising red flags.
"We make it very easy for individuals who are troubled or disturbed or want to engage in violent acts to get very powerful weapons very quickly," Obama said.
"And that's a problem," Obama said, as the White House demanded that the Republican-controlled Congress pass legislation to curb assault weapons like the one used in Orlando.
The White House said Obama would travel to Orlando on Thursday to "pay his respects to victims' families, and to stand in solidarity with the community as they embark on their recovery."
The FBI admitted it had previously investigated the 29-year-old American of Afghan descent, but had cleared him of extremist ties.
Relatives and acquaintances have painted a picture of a violent and unstable young man who had beat his ex-wife and expressed homophobic views.
Police have identified all 49 victims -- mainly clubgoers celebrating Latin Night at one of Orlando's most prominent gay venues at the start of Gay Pride month. Many of the wounded were still in hospital.
As the club was closing around 2:00 am, Mateen began a gun battle with an off-duty officer working at the club. More officers responded, and the gunman escaped to the bathroom, where he barricaded himself and seized hostages.
Witness Janiel Gonzalez described scenes of mayhem as the gunman sprayed revelers with bullets.
"It was like complete chaos," he told AFP. "People were screaming 'Help me, help me, I'm trapped!'"
Orlando Police Chief John Mina said he made the difficult decision to burst into the club after Mateen made comments in telephone calls about "bomb vests, about explosives."
"We knew that was the right thing to do and believe we prevented a future loss of life and saved many, many lives," Mina told reporters.
Police used explosives and a BearCat armored car to punch a hole in the wall and dozens more survivors surged out of the breach. Mateen opened fire and was cut down by police.
Officials in Orlando, a resort city famous worldwide as the home of Walt Disney World and other amusement parks, were stunned by the tragedy but vowed that the community would pull together.
"This is not a war zone that we are living in. This is a civilized society and we had to deal with something unthinkable," Orange County Sheriff Jerry Demings said.
The attacks triggered solidarity vigils by gay and lesbian groups -- and their supporters -- across the country and abroad.
Thousands of people marched under rainbow flags in London's Soho district, bursting into chants of "We're here, we're queer, we will not live in fear." More than 100,000 had paraded through Los Angeles on Sunday at a previously planned pride parade.
But no mood of national outrage could survive long in the crucible of America's 2016 presidential race, and presumptive Republican presidential nominee Donald Trump seized his chance to launch an attack on Democratic rival Hillary Clinton.
"Clinton wants to allow radical Islamic terrorists to pour into our country," he said, as he vowed to clamp down on immigration from nations with links to terror attacks targeting the United States or its allies.
Clinton pushed back by vowing to make identifying "lone wolves" a top priority and issuing a pointed warning to US allies in the Middle East, including Saudi Arabia, to crack down on citizens supporting extremism.
"The Orlando terrorist may be dead, but the virus that poisoned his mind remains very much alive," she said.
But Clinton also hit out at her Republican rival for suggesting American Muslims may be covering for extremists in their midst.
"That's wrong," Clinton said. "And it's also dangerous. It plays right into the terrorists' hands."