Norwegian world champion swimmer Alexander Dale Oen died of a suspected heart attack in Arizona at the age of 26, the Norwegian Olympic Committee said on Tuesday.

Alexander Dale Oen of Norway shows off his silver medal after the men's 200m breaststroke final at the European Swimming Championships in Budapest on August 12, 2010. [File photo]

Dale Oen became a national hero last year when he won the 100 metres breaststroke at the world championships in Shanghai just days after Norway had been rocked by the massacre of 77 people by far right fanatic Anders Behring Breivik.

One of his country's best hopes for a medal at this year's London Olympics, Dale Oen was attending an altitude training camp when he died.

After a day of light training and a game of golf, his team mates became worried when they noticed he had spent a long time in the shower on Monday evening and after breaking into the bathroom, they found him lying half-in and half-out of the bath tub.

Ola Ronsen, doctor to Norway's elite Olympians, was quickly on the scene and an ambulance arrived shortly afterwards, but despite their best efforts to revive him Dale Oen was pronounced dead at 2100 local time.

"This is incredibly sad and tragic," Ronsen said. "As a doctor, it is painful experience not to succeed with resuscitation.

"Everything was done according to procedure, and everything was tried, so it's infinitely sad that we were unable to revive him."

Born in Oygarden in south-western Norway, Dale Oen's career was ground-breaking for Norwegian swimming, and his bronze medal at the 2006 short course championships was the first for a Norwegian man in a world event.

He also won his country's first Olympic swimming medal when he took silver in the 100m breaststroke in Beijing in 2008 but it was at the 2011 world championships that he became a real national hero to the Norwegian people.

As he prepared for the 100m breaststroke competition in Shanghai, Norway was thrown into shock by the massacre of 77 people and Dale Oen struggled with his emotions in the aftermath, touching the Norwegian flag on his swimming cap every time he entered the pool deck.

Three days later, he swam to victory in the 100m breaststroke final and dedicated his triumph to the Norwegian people.

"We need to let everyday life come back because we cannot get things ruined," he told reporters after the race.

"In a time like this for Norway, we need to be together, to be one. I think now that everyone back at home, of course, is paralysed. I can feel the emotion, but I'm here in Shanghai and I have to show my best and ... just think of those at home."

Together with prime minister Jens Stoltenberg, the swimmer became a symbol of hope that life in Norway could return to normal after the savagery of Breivik's attacks.

These old wounds were recently ripped open once again when Breivik went on trial on April 16 in an Oslo courtroom.

The gruesome details of the 77 fatalities - the vast majority children and teenagers gunned down at a Labour youth camp on the island of Utoya - were once again played out across the media.

But instead of looking forward to the Olympics and another medal for Dale Oen, the small nordic nation is now preparing for another unexpected funeral.

Stoltenberg expressed his shock and grief at the loss of his compatriot, telling TV2 that "this is a great loss for his family and friends, but also for all of Norway".

He revealed that he had spoken to Dale Oen at the annual sports gala event in Norway.

"He told me that there was a strange, mixed feeling to win the medal while he received the tragic news from Norway, (but) the way he carried on and managed a great sporting performance, and also to show dignity, caring and compassion, it shows that he was not only a great athlete, he was also a warm and good person," Stoltenberg said.

Dale Oen was active on social media site Twitter and in his final message he told his followers he was looking forward to returning to his native country.

"2 days left of our camp up here in Flagstaff, then it's back to the most beautiful city in Norway - Bergen," he wrote.

Norwegian officials offered their condolences to Dale Oen's family and friends, and to the wider Norwegian sporting community.

"My thoughts go first and foremost to his family in Oygarden," Per Rune Eknes, president of the Norwegian Swimming Federation said in a statement. "This is the toughest day the sport of swimming in Norway has ever had."

"Norwegian sport has lost a sporting hero, not just because of his performances in the pool, but also because of his manner," said Borre Rognilen, president of Norway's Olympic council.