JK Rowling has tweeted her sadness after discovering one of the Orlando shooting victims worked at the Harry Potter ride in Universal Studios.

She posted: "Luis Vielma worked on the Harry Potter ride at Universal. He was 22 years old. I can't stop crying. #Orlando"

READ MORE: Orlando Pulse Club shooting: Fears death toll will rise after worst US mass shooting

It's feared the death toll from the deadliest mass shooting in US history will rise, as the identities of the 50 people who were gunned down began to emerge.

Seven men and one woman have been named as victims of security guard Omar Mateen, 29, who massacred revellers and at least one employee at a gay nightclub in Orlando, Florida.

More than 300 people were inside the Pulse nightclub at the time of the attack in the early hours of Sunday morning.

The killer, who also held hostages in a three-hour stand-off, died in a gunfight with Swat officers after they stormed the building. He had mortally wounded at least 50 victims and injured dozens more.

Officials said 39 people died in the venue and 11 at hospitals in the city where 53 people remain, most in a critical condition.

READ MORE: Orlando Pulse Club shooting: Fears death toll will rise after worst US mass shooting

Mateen was known to the FBI after making inflammatory comments made to co-workers about extremism in 2013. He was questioned twice and investigated again in 2014 over potential links to an American suicide bomber, but the probe was dropped after he was deemed to be low-risk.

However he was not under surveillance and the FBI said he had legally purchased two firearms within the last week.

The 911 calls involving the shooter from Port St Lucie, Florida that featured conversations about the Islamic State before the massacre have been handed to investigators.

Mateen is believed to have pledged allegiance to the Islamic State leader, Abu Bakr al-Baghdadi during the call.

The Islamic State's Amaq news agency is understood to have issued a statement saying the massacre was "carried out by an IS fighter".

However US officials said it was possible that he was self-radicalised but not "under the command and control of ISIS."

An ex-wife described Mateen, reportedly a regular at an Islamic centre in the city, as a violent body builder who had a history of abusing steroids.

READ MORE: Orlando Pulse Club shooting: Fears death toll will rise after worst US mass shooting

Meanwhile his father, Seddique Mir Mateen, told NBC news that his son had recently been angered after seeing two men kissing while out with his young son and denied the killings had a religious motive.

On Sunday evening vigils were held in Orlando and a group of around 100 people gathered in Paris in a show of solidarity.

British comedian James Corden, who hosted the Tony Awards in New York on Sunday evening, paid tribute to the victims and their loved ones saying "your tragedy is our tragedy".

David Cameron earlier 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 a personal message had been sent to President Obama, saying the Queen adn Prince Philip were "shocked by the events in Orlando.

"Our thoughts and prayers are with all those who have been affected."

Mr Obama 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.

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, 20, 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.

READ MORE: Orlando Pulse Club shooting: Fears death toll will rise after worst US mass shooting