Orlando Police via Twitter | Members of the Orlando emergency services seen around the Pulse nightclub after a fatal shooting and hostage situation on June 12, 2016.

A gunman killed 49 people at a packed gay nightclub in Orlando, Florida on Sunday in the worst mass shooting in US history, an attack that was later claimed by the Islamic State (IS) group.

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Family and friends of victims trapped at the Pulse nightclub waited anxiously on Monday to find out whether their loved ones were among the 49 people killed and 53 wounded in the deadly rampage.

The FBI and other law enforcement authorities were poring over evidence that could explain the motives for the massacre, which President Barack Obama denounced as an act of terror and hate.

The gunman, Omar Mateen, a New York-born Florida resident and US citizen who was the son of Afghan immigrants, was shot and killed by police who stormed the club with armored cars after a three-hour siege.

Mateen, 29, called emergency services during the shooting and pledged allegiance to the leader of the militant Islamic State (IS) group, officials said. His father said on Sunday his son was not radicalized, but indicated Mateen had strong anti-gay feelings. His ex-wife described him as mentally unstable and violent toward her.

Mateen exchanged gunfire with 14 police officers at the club and took hostages at one point. In addition to the assault rifle, he also had a handgun. Police Chief John Mina said officers held back for some time because Mateen indicated he had a bomb vest.

He opened fire at the Orlando club with an AR-15 semi-automatic rifle in such close quarters that the bullets could hardly miss. He was gunned down after police used explosives and a small armored vehicle to punch a hole in a wall and allow dozens of club-goers to escape, police said.

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, AP reported.

IS group claims responsibility

The IS group reiterated on Monday a claim of responsibility for the attack. "One of the Caliphate's soldiers in America carried out a security invasion where he was able to enter a crusader gathering at a nightclub for homosexuals in Orlando, Florida ... where he killed and injured more than a hundred of them before he was killed," the group said in a broadcast on its Albayan Radio.

Although the group claimed responsibility, this did not necessarily mean it directed the attack: there was nothing in the claim indicating coordination between the gunman and the jihadist outfit before the rampage.

Orlando gunman called 911 moments before shooting and pledged allegiance to leader of ISIS, sources tell NBC News https://t.co/0XkMoA2xo2 — NBC News (@NBCNews) June 12, 2016

Addressing the nation on Sunday afternoon, President Barack Obama said the nightclub shooting in Orlando was an "act of terror" and an "act of hate", a “sobering reminder” that an attack on any American, regardless of race, religion or sexual orientation, was an attack “on all of us”.

Presumptive Democratic presidential nominee Hillary Clinton also said the attack was an act of "terror" and "hate", expressing her solidarity with the LGBT community and pledging to "redouble our efforts to defend our country from threats at home and abroad".

Her Republican rival in the 2016 presidential race Donald Trump, who has called for Muslims to be banned from travelling to the United States, tweeted: "When will we get tough, smart and vigilant?" he demanded, later boasting of having been "right on radical Islamic terrorism".

Trump also reiterated his call, which was central in his campaign for the Republican nomination, to ban foreign-born Muslims from entering the country.

'Everyone get out of Pulse and keep running'

A police officer working as a security guard inside the Pulse nightclub, which has operated in downtown Orlando since 2004 and was hosting some 350 revellers, exchanged fire with the suspect at about 2am (0600 GMT), police officials said.

“Everyone get out of Pulse and keep running,” the club’s management wrote on Facebook as the incident unfolded.

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A hostage situation developed, and three hours later SWAT team officers used armoured cars to storm the club before shooting dead the gunman. It was unclear when the victims were killed.

Dozens of terrified patrons, some of whom had been hiding in restrooms, were rescued. One officer was injured when he was hit in his helmet while exchanging fire with the gunman, police said.

Video footage showed police officers and civilians carrying some people away from the club and bending over others on the ground. Dozens of police cars, ambulances and other emergency vehicles could be seen in the area.

Father asks 'Why'?

Despite Mateen's 911 call expressing support for the IS group, US officials said on Sunday they had no conclusive evidence of any direct connection with foreign extremists.

"So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre," said a US counterterrorism official. "This guy appears to have been pretty screwed up without any help from anybody."

Authorities said Mateen had been twice questioned by FBI agents in 2013 and 2014 after making comments to co-workers about supporting militant groups, but neither interview led to evidence of criminal activity

Ronald Hopper, the FBI's assistant special agent in charge on the case, said Mateen was questioned in 2014 about his contacts with Moner Mohammad Abu-Salha, a US citizen who also had lived in Florida and became a suicide bomber in Syria that

year.

'Our hearts are heavy'

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Pulse, which was crowded for a Latin music night, describes itself on its website as more than “just another gay club”. One of the founders and owners, Barbara Poma, opened it more than a decade ago in an effort to keep alive the spirit of her brother, who died after battling HIV.

The choice of target was especially heart-wrenching for members of the US lesbian, gay, bisexual and transgender community, LGBT advocacy group Equality Florida said in a statement.

“Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety,” the group said. “We will await the details in tears of sadness and anger.”

Orlando has a population of 270,930 and is the home of the famed Disney World amusement park and many other tourist attractions that attracted 62 million visitors in 2014.

It was the second deadly shooting at an Orlando night spot in as many nights. Late Friday, a man thought to be a deranged fan fatally shot singer Christina Grimmie, a former contestant on “The Voice”, as she was signing autographs after a concert.

Condolences and messages of support poured in from around the world.

French President François Hollande said he was “horrified” by what had happened, and pledged “the full support of France and the French people to the US authorities at this challenging time”.

"Our hearts are heavy," said German Chancellor Angela Merkel during a visit to China.

"We are determined to continue living in an open and tolerant way even if such murderous attacks plunge us into deep mourning," Merkel said.

(FRANCE 24 with AP, REUTERS)

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