The death of the 26-year-old, a bronze medallist in both the 10km event at the Rome World Championships in 2009 and the FINA World Open Water Championships in Roberval, Canada, this year, has sent shockwaves through the sport.

"While the causes of this death remain under investigation, FINA and the entire aquatic community address, in this moment of grief, their deepest and sincere condolences to his family, friends and to US Aquatic Sports and USA Swimming family," FINA said in a statement.

USA Swimming added that it was "deeply saddened by the death of Fran Crippen", who was a two-time US champion in the pool in the 800 metres freestyle event before moving into open water swimming in 2006.

Crippen, who was from Pennsylvania, had won the gold medal at the Pan American Games in Rio de Janeiro in 2007.

"Fran was a champion swimmer but more importantly, a tremendous person, and he will be remembered for so many extraordinary qualities," said USA Swimming

"We send our sincere condolences and deepest sympathies to the Crippen family, a family that is a treasured member of the USA Swimming family.

"We will continue to work with FINA, the meet host and others to determine how this tragedy occurred."

Organisers in the United Arab Emirates had called in deep sea divers after all swimmers were thought to have finished the five 2km laps.

The police and coast guard divers spent two hours searching for Crippen, managing to find him unconscious just before the final buoy, it reported.

He was brought to shore and rushed by ambulance to the main hospital in Fujairah, one of the seven Emirates that make up the UAE and the only one that lies on the Gulf of Oman.

Other swimmers claimed that Crippen, in contention for the World Cup in pursuit of eventual winner Chad Ho of South Africa, had showed signs of fatigue sometime on the third lap of the five-lap race.

"I can't even imagine how this could have happened to Fran," Crippen's team-mate Alexander Meyer said.

Bulgarian veteran Petar Stoychev added: "It is far too bizarre to think that such a thing could have happened."

Ayman Saad, the organising director from the UAE Swimming Association, said all activities related to the event had been cancelled.

"At the moment we are trying to complete the formalities with the police and medical staff to repatriate the body back to the United States," Saad said.

"This is a sad moment and we do not want to go into the details till the medical report is put forward."