Norwegian swimming champion Alexander Dale Oen has died in the US, aged 26, Norway's swimming federation says.
He was found collapsed in a shower late on Monday after training in Flagstaff, Arizona, Norwegian media say. Officials said he had suffered a cardiac arrest.
Emergency services arrived at the scene within minutes but were unable to revive him.
Dale Oen won gold in the 100m breaststroke at the World Championships in Shanghai in July 2011.
His triumph came just days after the attack in Norway by Anders Behring Breivik which killed 77 people.
Norwegian Swimming Federation President Per Rune Eknes said the swimmer had suffered a cardiac arrest. Friends said he had appeared healthy earlier.
Doctors at the Flagstaff Medical Center declared the world champion swimmer dead at 21:00 local time on Monday after all efforts to resuscitate him failed.
Per Rune Eknes told national broadcaster NRK that it was the blackest day in the history of Norwegian swimming.
"We are all in shock… our thoughts go primarily to his family who have lost Alexander way too early," said Norwegian Coach Petter Loevberg.
Dale Oen's last tweet on Monday, as he was coming to the end of training in the US, said: "2 days left of our camp up here in Flagstaff, then it's back to the most beautiful city in Norway #Bergen".'Mistake'
Olympic bronze swimming medallist and BBC Sport analyst Steve Parry said he could not believe the news.
"I thought it was a mistake - I mean such a fit guy, a world champion suffering a cardiac arrest," he said.
"He was one of the nicest people involved in our sport. He was a rock solid guy. Usually a lot of people in his position don't have that much time but he made sure he had time for everyone."
The BBC's Chris Mitchell, who was there when Dale Oen won gold in Shanghai, remembers the moment: "He was quick. His time of 57.81 seconds was the fourth fastest in history and a Norwegian record. It is unusual for a swimmer to win so convincingly over such a short distance but Dale Oen was inspired and led from start to finish."
Dale Oen was born in Bergen, Norway's second largest city, and began swimming at the age of four. He got his international breakthrough in 2005 when he came seventh in the 100m breaststroke during the World Aquatics Championships in Montreal, Canada.
He won silver at the 2008 Olympics in Beijing, Norway's first Olympic swimming medal, and was considered a strong hope for this summer's London Games.
After winning his gold medal in Shanghai, he said that the murders of 77 people in Oslo and on the island of Utoeya had affected him deeply. "We can't let this guy ruin the future for us," he said.
Alexander Dale Oen brought home Norway's first Olympic swimming medal, winning silver in Beijing 2008.
More geared towards winter sports, Norway is not traditionally renowned for swimming. He has been a great inspiration to aspiring swimmers across the country, many of whom struggle with poor facilities and funding.
Add to this Dale Oen's young age and his famously friendly personality, and his premature death has come as a great shock to the whole country.
Norwegian Prime Minister Jens Stoltenberg summed up what many Norwegians will be thinking: "A small country has lost a great athlete."
Speaking to Dale Oen's local paper, Bergens Tidende, the prime minister said the swimmer had impressed the whole world in Beijing and was one of Norway's great hopes in London.
Norway's Prime Minister Jens Stoltenberg expressed his sorrow at the death of the swimmer.
"Alexander Dale Oen was a great athlete for a small country. My thoughts go to his family and friends," he said.