Dale Oen, who had been considered one of his country's best hopes for a medal at this year's Summer Olympic Games in London, collapsed at a training camp in Flagstaff in northern Arizona on April 30 and was later pronounced dead. He was 26.
A first autopsy in early May into the swimmer's death proved inconclusive. The results of a second set of tests released Tuesday by the Coconino County Public Health Services District found he died of atherosclerotic coronary artery disease.
The condition is essentially a hardening of the arteries caused by plaque build-up that narrows and makes the arterial passages stiffer. The condition makes it harder for blood to flow through arteries and is a common cause of heart attack.
"Based on the autopsy findings and the investigative history that is available to me, it is my opinion that Alexander Dale-Oen died of atherosclerotic coronary artery disease," Coconino County medical examiner Lawrence Czarnecki wrote in a report.
The second tests revealed "severe atherosclerotic disease of the coronary arteries" supplying blood to the heart, and found that the left descending artery was "occluded" by "atherosclerotic plaque."
"The decedent's only known risk factor for heart disease was familial," Czarnecki found, noting that one of Dale Oen's grandfathers died suddenly of heart disease at age 42.
"Given the decedent's young age and significant atherosclerotic disease, a follow-up and evaluation of family members is recommended," he added.
Dale Oen was found lying partially in a bath tub at the training facility after a day of light training and a game of golf. His team-mates broke into the bathroom after noticing he had spent a long time inside.
A team doctor and paramedics tried unsuccessfully to revive him.
He became a national hero in Norway last year when he won the 100 meters breaststroke at the world championships in Shanghai.