"The autopsy report shows that Dale Oen had a coronary artery illness, meaning narrow blood vessels in the heart muscle due to serious atherosclerosis," Olympiatoppen, which is responsible for Norwegian elite sport training, said in a statement published on the Aftenposten daily's website.
The autopsy — conducted in the United States after the swimmer died on April 30th at a training camp in Flagstaff, Arizona ahead of the London Olympics — had also shown that Dale Oen suffered the equivalent of small heart attacks one to two months before his death, the statement added.
"This is a very surprising finding, and it is almost incomprehensible that a 26-year-old world swimming champion can have developed such a serious heart disease," Olympiatoppen physician in charge of swimming, Ola Roensen, said in the statement.
Roensen said the autopsy report findings meant a shoulder injury that plagued Dale Oen earlier in the year and that had been believed to be due to a pinched nerve, was probably linked to the heart disease.
Eivind Berge, the chief physician at the cardiovascular division at the Oslo university hospital, said in the statement that it was a shame intense medical tests of the swimmer's shoulder injury had failed to detect his heart condition.
"If there had been a suspicion of coronary artery disease, it probably would have been possible to detect the illness and prevent his death," he said.
Dale Oen's family meanwhile said they were "grateful to have clarified the reason for Alexander's death."
The swimmer had collapsed in the bathroom of his hotel room after a training session and was found after his teammates thought he was taking an unusually long time, and became worried when there was no reply when they knocked on the door.
The team doctor had tried to resuscitate him, as did ambulance paramedics who arrived within minutes to take him to hospital, but to no avail, and he was declared dead at the hospital.
Dale Oen became Norway's first swimmer to win an Olympic medal when he won the silver in the 100m breaststroke in the 2008 Games in Beijing.
He won the gold medal in the 100m breaststroke in the 2011 World Championships in Shanghai and had been tipped as one of Norway's best chances at a medal at the London Olympics in July.