The so-called Islamic State (IS) have claimed responsibility for the deadliest shooting in US history, as it emerged that the suspected gunmen has been known to law enforcement agencies since 2013.
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, killing at least 50 people. A further 53 people are also in hospital, with most in a critical condition.
"I think we will see the death toll rise," said Dr Mike Cheatham from the Orlando Regional Medical Center.
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
While IS on Sunday claimed involvement in the shooting - the biggest mass killing of LGB people in the West since the Holocaust - through its Amaq news agency, the extent of their involvement is unclear, and the FBI and other commentators have said the link requires further investigation.
IS said of the attack: "The armed attack that targeted a gay night club in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter."
Three US officials familiar with the investigation into the massacre said that no evidence had yet been found showing a direct link with IS or any other militant group, Reuters reported.
There is “no evidence yet that this was directed or connected to ISIS. So far as we know at this time, his first direct contact was a pledge of bayat (loyalty) he made during the massacre,” a US counter-terrorism official, referring to a 911 call the suspect made on Sunday. The official spoke on condition of anonymity.
A US intelligence official said it was not unexpected that IS would claim responsibility given that the group has been suffering serious losses of fighters and territory in Iraq and Syria, the news agency reported.
“The fact that a website connected to Daesh applauded it doesn’t mean anything,” the US intelligence official was further quoted as saying.
“They are losing on their home turf, and it’s not surprising if they’re looking for some kind of twisted victory.”
CNN's Salma Abdelaziz, who the broadcaster said closely monitors IS messaging, cautioned about taking the message at face value.
She said the message was inconsistent with previous IS announcements and that the Arabic word for gay was used rather than an epithet normally used by the terror group. Also, she said there was no claim that the attack was directed, just an after-the-fact claim the gunman was an IS fighter.
The New York Times' Rukmini Callimachi wrote on Twitter that IS uses the internet to target and recruit the mentally ill and that attacks carried out in their name play out in a familiar way.
The MailOnline reported that in recent months IS had been trying to inspire "lone wolf" attacks, including publishing a kill list with the names of several hundred Florida residents on it.
The website said IS published the list in April on anonymous messaging app Telegram.
Mateen's father, Seddique Mir Mateen, has since insisted the killing spree "had nothing to do with religion" and that a recent experience in which his son saw two men kissing may have prompted the shooting which he apologised for.
The killer's ex-wife, Sitora Yusufiy, has said she suspects Mateen was "mentally ill".
Yusuify lived with Mateen for four months in 2009, before her family "rescued" her from the relationship after becoming aware she was being physically abused.
The BBC quoted her as saying: "When he would get in his tempers, he would express hate toward everything. He was mentally unstable and mentally ill: that's the only explanation that I could give."
Mateen's 911 calls were said to have featured conversations about IS and it is believed he pledged allegiance to its leader, Abu Bakr al-Baghdadi. Records of the calls are now become federal evidence.
FBI special agent Ronald Hopper was quoted by USA Today as saying agents had investigated Mateen in 2013 and again in 2014 regarding terror threats, but lacked sufficient evidence in both cases to pursue charges.
Mateen, 29, first came to the attention of authorities following inflammatory comments made to co-workers, then again over ties to an American suicide bomber, but he was not under FBI surveillance.
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.
A security company Mateen had previously worked for, G4S Secure Solutions, has also since come out to say it vetted him twice, once in 2007, and again in 2013. He had carried a gun as part of his job.
The FBI said Mateen, who was born in New York was an American citizen, legally purchased two firearms within the last week.
Meanwhile, the names of those killed in the attack have began to be released.
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.
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.
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 condemning 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.