Identities of the 50 people mown down in the deadliest mass shooting in US history continue to emerge amid fears the death toll from the "act of terror" may rise.
Omar Mateen was armed with a powerful assault-type rifle and handgun when he sprayed revellers with bullets at the popular gay nightclub, Pulse, in Orlando, Florida.
The FBI have said the 29-year-old killer born in New York was an American citizen who legally purchased two firearms within the last week before murdering at least 50 people.
Edward Sotomayor Jr, 34, Stanley Almodovar III, 23, Luis Omar Ocasio-Capo, 20, and Juan Ramon Guerrero, 22, were among the first victims to be named.
Peter O. Gonzalez-Cruz, 22, and Eric Ivan Ortiz-Rivera 36, and Luis S Vielma, 22, have since been added to the list of deceased.
More than 300 people were inside the building at the time of the attack - 53 people are also in hospital, with most in a critical condition.
Despite being aware of Mateen since 2013, following inflammatory comments made to co-workers and over ties to an American suicide bomber in 2014, he was not under surveillance by the FBI.
Authorities deemed his link to the bomber to be minimal and that he did not constitute a threat, and after interviews and an investigation dropped the probe into his comments.
It has been revealed that 911 calls involving the shooter from Port St Lucie, Florida and featuring conversations about the Islamic State before the massacre, have now become federal evidence.
He is believed to have pledged allegiance to the Islamic State leader, Abu Bakr al-Baghdadi.
US president Barack Obama called the killings at the gay-friendly establishment an "act of terror" and an "act of hate" and said they are being investigated as terrorism.
He praised the emergency service response and described the gunman as a person "filled with hatred".
President Obama said the massacre is a reminder of how easy it is for someone to access a weapon like a gun, allowing them to go on and shoot other people.
"We have to decide if that is the type of country we want to be. To actively to do nothing is a decision as well," president Obama added.
Seddique Mir Mateen, the father of the shooter, told NBC News that he thinks a recent experience in which his son saw two men kissing might be related to the shooting.
He apologised for the whole incident, said he was not aware of the actions of his security guard son and insisted "this had nothing to do with religion".
The killer, who also held hostages in a three-hour stand-off, later died in a gunfight with Swat officers after they stormed the building.
The majority of the patients were taken to the Orlando Regional Medical Center were in critical condition, according to a trauma surgeon there.
"I think we will see the death toll rise," Dr Mike Cheatham said.
Mr Obama has ordered flags at the White House and federal buildings to be flown at half-mast as a mark of respect for the victims. The Empire State Building in New York was left in darkness as a mark of sympathy on Sunday evening.
And in the aftermath of the massacre, police departments across the US increased patrols around popular gay-friendly locations and venues.
Orlando Mayor, Buddy Dyer, described the scene saying there was "blood everywhere".
The Prime Minister joined political leaders from around the world in condemnation of the attack and offered their condolences to the victims.
David Cameron said he was "horrified" by the shooting, while Afghanistan's president, Ashraf Ghani said "targeting civilians is not justifiable under any circumstances whatsoever".
French president Francois Hollande said he "expresses the full support of France and the French with America's authorities and its people in this difficult time".
And Buckingham Palace said the Queen had sent a personal message to President Obama, saying: "Prince Philip and I have been shocked by the events in Orlando.
"Our thoughts and prayers are with all those who have been affected."
In the wake of the slaughter, a heavily-armed Indiana man was arrested on his way to a southern California gay pride parade.
Police stopped James Wesley Howell and discovered an arsenal of weapons in his vehicle including three assault rifles, high-capacity magazines and ammunition, and a five-gallon bucket with chemicals that could be used to make an explosive device.
The 20-year-old was arrested at around 5am on Sunday and said he wanted to do harm at the event that draws in crowds of thousands.