THE gunman who killed 49 people in an attack on a gay nightclub in Orlando was a "homegrown extremist" who supported a number of different Islamist groups, according to the FBI and the White House.
Counter-terrorism authorities have been investigating the background of Omar Mateen and defended their handling of the gunman after it was revealed they had had previous contact with him.
Mateen, a 29-year-old American-born Muslim, opened fire at the crowded Pulse Orlando club early on Sunday.
He was killed in a gun battle with a SWAT team after police used explosives and a small armoured vehicle to punch a hole in a wall and allow dozens of revellers to escape.
Orlando's Mayor Buddy Dyer said: "We will not be defined by the act of a cowardly hater."
FBI director James Comey said Mateen had "strong indications of radicalisation" and was probably inspired by foreign terrorist organisations.
He said Mateen called the 911 emergency number around the time of the attack and pledged loyalty to the Islamic State as well as solidarity with the Tsarnaev brothers who carried out the Boston Marathon bombing. He also expressed solidarity with a suicide bomber who died on behalf of the al-Nusra front, a group which is at odds with Islamic State.
Mateen had also expressed support in recent years for both al Qaida and its enemy Hezbollah, Mr Comey said.
The FBI investigated Mateen for 10 months beginning in May 2013 after he was said to have made inflammatory remarks in support of terrorists.
Mr Comey said investigators introduced him to confidential sources, followed him and reviewed some of his communications, but Mateen claimed he made the remarks in anger because co-workers were teasing and discriminating against him because he was Muslim. The investigation was brought to a close.
Asked about whether the FBI should have done anything differently, Mr Comey said: "So far the honest answer is, 'I don't think so."
At the White House, US President Barack Obama said there is no clear evidence so far that Mateen was directed by the Islamic State. He said Mateen was inspired by radical information over the internet, calling it another apparent example of "homegrown extremism".
Meanwhile, a steady stream of people filed through a makeshift memorial about a mile from the nightclub. It consisted of dozens of bouquets and candles.
Counter-terrorism experts have been warning in the past few years about the danger of so-called lone wolf attackers who act in sympathy with extremist groups like the Islamic State but are not directed by them.
Despite the 911 call from the club, Mateen's intentions seemed to become murkier when his Afghan immigrant father suggested anti-gay hatred was another possible motive. The father said his son got angry a few months ago when he saw two men kissing in Miami.
Mateen's ex-wife attributed the violence to mental illness, saying he was bipolar and abusive towards her.
Mr Obama said investigators are still looking into the killer's motivations and considering all possibilities, noting that Muslim extremist groups like the Islamic State have been known to target gays.
The Islamic State's radio called Mateen "one of the soldiers of the caliphate in America". Al-Bayan Radio, a media outlet for the extremist group, hailed the attack, saying that it targeted a gathering of Christians and gays and that it was the worst attack on US soil since the events of September 11 2001.
The statement gave no indication of whether the group planned or knew of the attack beforehand.
Mateen's father, Seddique Mir Mateen, told reporters that the massacre was "the act of a terrorist", and added: "I apologise for what my son did. I am as sad and mad as you guys are."
Asked whether he missed his son, he said: "I don't miss anything about him. What he did was against humanity."
Thirty-nine of the dead were killed at the club, and the others died at hospitals, the mayor said. Authorities were still notifying victims' families on Monday.
At least 53 people are being treated in hospital, including five in grave condition, meaning the death toll could rise.
The previous deadliest mass shooting in modern US history was the 2007 attack at Virginia Tech, where a student killed 32 people and took his own life.
Mateen bought at least two guns legally within the last week or so, according to Trevor Velinor of the Bureau of Alcohol, Tobacco and Firearms.
Mateen exchanged gunfire with 14 police officers at the club and took hostages at one point. In addition to the assault rifle and handgun, he had a weapon in his vehicle.
Police Chief John Mina said officers held back for some time because Mateen indicated he had a bomb vest. At about 5am, authorities sent in a SWAT team to rescue the remaining club-goers.
