Islamic State (IS) terrorist group has claimed the responsibility for the shootings in which at least 50 people were killed and 53 others injured when a "lone wolf" gunman opened fire early on Sunday in a gay nightclub in Orlando, Florida, where a state of emergency has been declared.
In a message published on the group’s semi-official news agency, Amaq, it described gunman Omar Mateen as a “soldier of the caliphate”, The Telegraph (UK) reported.
Although the statement did not clarify Mateen’s relation to the group, but the language appeared to suggest he was viewed as a lone wolf attacker.
The gunman Omar Mateen, 29, was killed by a SWAT team after he took hostages at Pulse, a popular gay night club.
It is believed that the suspect, who was a US citizen from the Florida town of Port St Lucie and was of Afghan descent, was not on a terrorism watch list, although he was being investigated for an unrelated criminal act, BBC reported.
Officials said the killings were likely to be ideologically motivated, though there was no information that the gunman was associated with a particular group.
He was armed with an assault rifle, a handgun and an unspecified "device", said officials.
"It appears he was organised and well-prepared," Orlando Police Chief John Mina said.
The shooter's father told NBC News: "This has nothing to do with religion."
Mir Seddique said Omar Mateen had been angry when he saw two men kissing in Miami recently.
"We are saying we are apologising for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country," Seddique told NBC News .
US President Barack Obama was briefed on the mass shooting, which police described as an act of terrorism.
"Our thoughts and prayers are with the families and loved ones of the victims. The President asked to receive regular updates as the FBI, and other federal officials, work with the Orlando Police to gather more information," said a statement by Barack Obama's press secretary.
US Congressman Alan Grayson said it was "no coincidence" the attack happened in a gay club.
Relatives were gathering at local hospitals desperate for news of their loved ones.
Many had received calls and texts from loved ones inside the club as the siege began, and some have heard nothing since.
The death toll given by Mayor Dyer means that the Orlando attack surpasses the 2007 massacre at Virginia Tech which left 32 people dead.
"Today we are dealing with something that we never imagined and is unimaginable," he said, adding that there was "an enormous amount of havoc" and "blood everywhere".
"Because of the scale of the crime I've asked the [Florida] governor to declare a state of emergency," he said.
"We're also issuing a state of emergency for the city of Orlando so that we can bring additional resources to bear to deal with the aftermath."