Speaking from the Oval Office in Washington DC after a meeting with the FBI Director James Comey, Homeland Security Secretary Jeh Johnson and other officials, Obama said the attack appeared to be similar to a shooting late last year in San Bernardino. "But we don't yet know," he cautioned.
Last December, 14 people were killed and 22 seriously injured in a mass shooting and attempted bombing in San Bernadino, California.
The self-styled "Islamic State" (IS) terror group claimed responsibility for the attack in Orlando, saying in a radio bulletin that it was carried out by "one of the soldiers of the caliphate." But US officials maintain that the 29-year-old shooter, Omar Mateen, had no known direct links to IS.
Obama said an initial probe into that attack had reported that the killer hadn't been directed from overseas but had rather been "partly inspired" by the group. The investigation is being treated as a terror probe, with investigators examining materials from the internet that the shooter may have read.
"At this stage, we see no clear evidence that he was directed externally," Obama said. "It does appear that at the last minute, he announced allegiance to ISIL," he said, using another acronym for IS.
Obama said that their "vicious, bankrupt ideology" and their religious beliefs about homosexuality led organizations like IS, al Qaeda and others to target gays and lesbians. "The fact that it took place at a club frequented by the LGBT community I think is also relevant," Obama said.
He added that Mateen did appear to have been "inspired by various extremist information that was disseminated over the internet."
FBI chief Comey later told reporters that the shooter appeared to have "drawn
inspiration from a diverse and complex web of competing terrorist groups," although he had no known ties to any terrorist group.
Comey cited the al-Nusra Front and Hezbollah as other groups that might have influenced the shooter besides IS.
Reaction on the campaign trail
In response to the Orlando attack, Donald Trump has pledged to suspend immigration from countries "where there is a proven history of terrorism" against the US.
Following last year's terror attacks in Paris, the presumptive Republican Party presidential nominee called for Muslims to be banned from entering the country. He reiterated that call on Monday, telling an audience in New Hampshire that radical Muslim immigrants were "trying to take over our children and convince them how wonderful ISIS (Islamic State) is."
Meanwhile, his Democratic party rival Hillary Clinton struck a more measured tone, stressing that such proposals would only make it more difficult for law enforcement to work with Muslim communities.
"Inflammatory anti-Muslim rhetoric and threatening to ban the families and friends of Muslim Americans, as well as millions of Muslim business people and tourists from entering our country, hurts the vast majority of Muslims who love freedom and hate terror," she said.
Clinton also called for tougher gun control measures, and attacked the Republican-controlled Congress for what she called a "totally incomprehensible" refusal to address the country's lax gun laws.
"We can't fall into the trap set up by the gun lobby that says if you cannot stop every shooting you shouldn't try to stop any," she said.
Trump, on the other hand, has set himself up as a champion of the Second Amendment [the US constitutional ammendment guaranteeing a "well-regulated militia" and the "right to bear arms" - the ed.] and dismissed calls for greater gun control.
jbh,nm/msh (dpa, AP, Reuters, AFP)