ORLANDO • The man who killed 49 people at a gay nightclub in Florida appears to have acted alone, without direction from the various Islamist militant groups he professed sympathy for, the authorities said as they delved into the roots of the worst mass shooting in modern US history.

Federal law enforcement officials said Omar Mateen, who worked as a private security guard at a gated retirement community, seemed to have been largely inspired by radical ideology he was exposed to over the Internet. US President Barack Obama called Omar, a New York-born US citizen and son of Afghan immigrants, an apparent example of "home-grown extremism".

In the midst of his rampage on Sunday, Omar placed a series of calls to emergency 911 dispatchers in which he pledged loyalty to Islamic State in Iraq and Syria leader Abu Bakr al-Baghdadi. He also claimed solidarity with the ethnic Chechen brothers who carried out the 2013 Boston Marathon bombing, and with a Palestinian-American who became a suicide bomber in Syria for the Al-Qaeda offshoot known as the Nusra Front, the authorities said.

"As far as we can tell right now, this is certainly an example of the kind of home-grown extremism that all of us have been so concerned about for a very long time," Mr Obama said at the Oval Office on Monday.

He added that the mass shooting in Orlando was being investigated as an act of terrorism.

The Federal Bureau of Investigation (FBI) said Omar had expressed sympathy for a variety of Islamist extremists, including groups in the Middle East that are sworn enemies. FBI director James Comey said: "It's not entirely clear at this point just what terrorist group he aspired to support."

REUTERS