A survivor of the massacre in Orlando, Florida in the US has described how the gunman returned to people he had already shot to make sure they were dead.
The attack at gay nightclub Pulse left 49 people dead and dozens wounded. The gunman was also killed by police.
The attacker, Florida resident Omar S Mateen, had pledged allegiance to the Islamic State militant group shortly before the attack, US authorities have said.
Six people remain in critical condition following the attack, hospital staff said.
Trauma surgeon Michael Cheatham said he "would not be surprised" if the death toll rose.
Speaking from a wheelchair, Angel Colon said he was shot three times in the leg and could not run.
"I can hear the shots come closer and I look over and he shoots the girl next to me. And I'm just there laying down and I'm thinking, I'm next, I'm dead.
"So, I don't know how but by the glory of God he shoots towards my head but it hits my hand so I just stay there, laying down, so he won't know that I'm alive."
Mr Colon said that, at times, the gunman was firing at people who appeared to be already dead.
He said Mateen was calm as he shot person after person in different rooms in the club.
"This person had to be heartless, ruthless," he said. "I don't know how he could do something like this."
According to NBC and ABC News reports, Mateen's wife told police she tried to talk her husband out of carrying out the attack.
Noor Zahi Salman has told investigators she drove him to the Pulse nightclub to check it out, US television reported.
She was also with him when he purchased ammunition.
The FBI has said it was investigating whether Mateen had been scouting out other potential targets, including Disneyworld.
Orlando police said they had no information yet regarding rumours Mateen had visited the nightclub before.
Mateen was reported to have been a regular visitor to the venue over a long period.
Chris Callen, a performer at Pulse, told the New York Daily News that Mateen had visited the venue several times.
Another man, Kevin West, told the Washington Post he had known Mateen through the dating app Jack'd and had recognised him as he walked into Pulse in the early hours of Sunday.
Media have quoted club customers who said they had seen him there, drinking a lot, up to a dozen times.
One said he had seen the killer "trying to pick up men".
The police said it was uncertain whether he would have been there scouting out targets or for another reason.
Meanwhile, US President Barack Obama has hit back at criticism of his anti-terrorism strategy by Republican presumptive presidential candidate Donald Trump in the wake of the Orlando massacre.
Mr Trump has repeatedly called for the President to use the term radical Islam when talking about terrorism.
He maintains many Muslim immigrants simply want to immigrate to kill Americans.
Mr Obama said the Republican candidate's proposed ban on Muslims travelling to the US would fuel extremist propaganda and make the country less safe.
"That's the key, they tell us. We can't beat this unless we call them radical Islamists. What exactly would using this label accomplish? What exactly would it change?"
"Groups like ISIL and Al-Qaeda want to make this war a war between Islam and America, or between Islam and the west.
"And if we fall into the trap of painting all muslims with a broad brush and imply that we are at war with an entire religion, then we are doing the terrorists' work for them."
Mr Obama questioned whether Republican Party officials agreed with the idea of a religious test at the border.
Mr Obama said calling a threat by a different name did not make it go away.
He said Islamic State fighters were not religious warriors, but thugs and thieves.
Mr Obama previously said it was appropriate for the FBI to investigate the massacre as an act of terrorism and said he would "spare no effort" to determine whether the assailant had been inspired or associated with an extremist group.
He also said the shooting was a reminder of how easy it was to get access to weapons that would allow such mass killings.
US Homeland Security Secretary Jeh Johnson has called for "meaningful, responsible" gun control measures in the wake of the killings.
He said such controls were a public safety issue and would not infringe on the rights of responsible gun owners.
Democrats were trying to push through a bill banning anyone convicted of a misdemeanour hate crime from owning a firearm.
FBI Director James Comey said there were "strong indications of radicalisation and of potential inspiration by foreign terrorist organisations".