Orlando nightclub killer Omar Mateen had expressed sympathy for a variety of Islamist extremists, including groups in the Middle East that are sworn enemies, the FBI said on Monday, as a picture began to emerge of the angry, violent man who carried out America's deadliest mass shooting.

U.S. authorities said they had found no direct links between Islamic State and Mateen, the U.S.-born son of Afghan immigrants who killed 49 people at a gay nightclub in Orlando on Sunday.

Mateen, 29, was shot dead by police who stormed the Pulse club with armoured cars after a three-hour siege. In 911 calls during his rampage, the killer expressed allegiance to Islamic State leader Abu Bakr al-Baghdadi.

But Federal Bureau of Investigation Director James Comey

said Mateen had made comments favourable to multiple armed Islamist movements and people, which "adds a little bit to the confusion about his motives." President Barack Obama said Mateen was likely a homegrown extremist.

"So far, 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," Comey told reporters in Washington. "We're highly confident this killer was radicalised at least in some part through the internet."

Islamic State, which controls territory in Iraq and Syria, reiterated on Monday a claim of responsibility, although it offered no signs to indicate coordination with the gunman.

In calls to authorities on Sunday, Mateen also mentioned support for the Boston Marathon bombers and a Florida man who became a Nusra Front suicide bomber in Syria, Comey said. Nusra is an al Qaeda offshoot which is at odds with Islamic State in Syria's civil war.

Co-workers reported Mateen to the FBI in 2013 after he had made "inflammatory and contradictory" statements, including a claim that he had family connections to al Qaeda and membership of Shi'ite militant group Hezbollah, a bitter rival.

The FBI's Miami office investigated Mateen for 10 months and interviewed him twice but found no evidence of a crime or connection with a militant group. Comey said the FBI was also "working to understand what role anti-gay bigotry may have played" in the attack.

The massacre reverberated on the presidential campaign trail, where Republican Donald Trump and Democrat Hillary Clinton, the two likely opponents in the Nov. 8 election, clashed over how to confront violent Islamist extremists.

Trump proposed suspending immigration to the United States from countries with a history of terrorism against America, Europe or U.S. allies, while Clinton warned against demonising Muslims and called for tougher gun safety measures.

Obama is to visit Orlando on Thursday to pay respects to families of the victims.

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