A survivor of the Orlando attack who was shot three times has described playing dead while Omar Mateen fired relentlessly at people on the floor.
Angel Colon told reporters Mateen had continued to shoot at club-goers who already appeared dead.
Six people remain in critical condition following the attack at the Pulse gay night club, hospital staff say.
Trauma surgeon Michael Cheatham said he "would not be surprised" if the death toll, currently at 49, rose.
Mateen, who was killed by police, pledged allegiance to so-called Islamic State (IS) shortly before the attack, authorities say.
US media reported on Tuesday that Mateen's wife had gone with him purchase ammunition and scope out the venue on a previous occasion.
The attack, in which 53 were wounded, is the worst mass shooting in recent US history.
Pulse attack survivor recounts night of Orlando shooting
Colon, who was shot in the leg, hand, and hip, told reporters he remembered thinking "I'm next, I'm dead", as Mateen fired again towards his head.
He said Mateen had shot dead a girl who was lying next to him.
Appearing in a wheelchair, he thanked the hospital staff who, he said, had been constantly by his side, and told them: "I will love you guys forever."
Colon 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."
Dr Cheatham said a number of the dead had gunshot wounds to the head. One victim who was shot in the head was in intensive care, he added.
Another hospital employee described fellow doctors moved to tears by the extent of the injuries.
Mateen had visited the Pulse club as a guest several times over the past three years and interacted with other club goers on gay dating apps, witnesses said.
Chris Callen, a performer at Pulse in Orlando, Florida, 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.
"I remember details," said West. "I never forget a face."
Investigators are questioning Mateen's wife, who reportedly accompanied the killer on a trip to survey the club before the shooting.
According to NBC and ABC News reports, Noor Zahi Salman told police she tried to talk her husband out of carrying out the attack.
Addressing reporters on Tuesday, President Obama said the country's thoughts were with survivors and relatives of the dead, as well as the LGBT community.
"You are not alone. The American people and our allies and friends all over the world stand with you," he said.
President Obama will travel to Orlando on Thursday.
Jeh Johnson, the Director of Homeland Security, said on Tuesday that the shooting highlighted the need for "meaningful, responsible" gun control measures.
Johnson said such controls were a public safety issue and would not infringe on the rights of responsible gun owners.
"This has become a matter of homeland security," he told CBS News. "We need to do something. We need to minimize the opportunities for terrorists to get a gun in this country."
FBI Director James Comey said there were "strong indications of radicalisation and of potential inspiration by foreign terrorist organisations".