The 2011 world 100 metres breaststroke champion and one of Norway's best hopes for a medal at this year's London Olympics was attending an altitude training camp in Flagstaff, Arizona.
His team-mates became worried when they noticed he had spent a long time in the shower on Monday evening and Dale Oen did not react when they called out and knocked on the door. When they got 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, Dale Oen's nordic homeland was rocked by the massacre of 77 people by far right fanatic Anders Behring Breivik.
The country was thrown into shock and Dale Oen struggled with his emotions in the aftermath of the attacks, 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."
Dale Oen, who was in Arizona preparing for the London Olympics, 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.