The gunman in a terrorist attack on a gay nightclub in Orlando, Florida, appeared to have been self-radicalized from Islamist material online, US President Barack Obama said Monday.
Obama told reporters that there is "no clear evidence that he was directed externally" or that the attack on the Pulse nightclub was part of any broader terrorist plot.
The president will travel Thursday to Orlando "to pay his respects to victims' families, and to stand in solidarity with the community as they embark on their recovery," the White House said later Monday.
Law enforcement officials in Florida said Omar Mateen, a 29-year-old US citizen who lived in the state, pledged allegiance to the Islamic State terrorist group in an emergency call while the attack was under way.
The FBI said 48 of the 49 people killed in the massacre on Sunday had been identified.
Several thousand people attended a vigil to honour the victims late Monday in Orlando.
FBI Director James Comey said the attacker appears to have drawn inspiration from a diverse set of terrorist groups, including Islamic State, but also competing groups like al-Nusra Front, and the Lebanese Shiite militia Hezbollah, who are all fighting each other in Syria.
The complex web of terrorist influences "adds to confusion about his motive," Comey said.
Investigators revealed details Monday of the siege at the Pulse nightclub.
Police chief John Mina said officers acted "heroically" in their first response to the gunfire, which erupted early Sunday as the club's Latin-themed night was drawing to a close. Around 300 people were estimated to have been in the building.
"Dozens and dozens of people were saved" from the mayhem as police rushed into the building and engaged in a gun battle with the shooter.
After Mateen moved deeper into the club, he barricaded himself in a bathroom with "four to five" hostages for several hours, Mina said. Fifteen to 20 people were hiding in a separate, opposite bathroom.
At one point Mateen, whose parents are immigrants from Afghanistan, called emergency operators and declared "an allegiance to Islamic State," Mina said.
Al-Bayan radio, which broadcasts inside Islamic State territory in Iraq, Syria and Libya, claimed Monday that the attack was carried out by a "soldier of the caliphate."
"God helped Omar Mateen" carry out an attack against "crusaders," said the morning broadcast on what is seen as the sanctioned station of the extremist group. It described Pulse as a "sodomite nightclub."
The attacker, whom Comey declined to name but who has been identified by Florida law enforcement, had been investigated by the FBI twice before, including for remarks he made to coworkers in 2013 claiming affiliations with al-Qaeda and Hezbollah.
In 2014, his name surfaced again during an investigation of another Florida man who had died fighting for al-Nusra, which is a rival to Islamic State, Comey said.
In calls with police during the attack, Mateen praised the Boston Marathon bombers and the al-Nusra fighter, and was "cool and calm" while on the phone with hostage negotiators, Mina said. The shooter said he was "wearing a vest" and had explosives, the police chief said.
Three hours after the worst mass shooting in US history began, the decision was made to breach the wall of the bathroom - first with explosives and then with an armoured vehicle - because Mina felt that "a further loss of life was imminent."
Mateen then came out of the bathroom and shot at officers, who returned fire and killed him.
The gunman legally purchased two firearms used in the attack - an AR-15 assault rifle and a handgun - within the last week, authorities said. A third gun was found in his van parked outside the club.
Mateen was employed as a private security guard with GS4 since 2007. As part of his job he carried a weapon, the company said.