Authorities suspected that Dale Oen, the world 100 meters breaststroke champion, suffered a heart attack on Monday at an altitude training camp in Flagstaff in northern Arizona.
An autopsy conducted on Tuesday for the Coconino County Public Health Services District found no obvious "anatomic" cause of death, or any associated trauma.
Barbara Worgess, chief health officer for the Coconino County Public Health Services District, said additional tests and consultation with a cardiovascular pathologist had been ordered, but that results would not be made public for at least eight weeks.
"Whenever there's no physical-anatomical obvious cause of death, it really becomes a process then of gathering more information and (seeing) where that information leads you," Worgess told Reuters.
She said consulting with a cardio-vascular expert was "just a good place to start" the further investigation.
"The important thing right now for the medical examiner is to keep a totally open mind, gather whatever information he can through tests and consultations ... and see where that takes us," she said.
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 teammates 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.
Dale Oen was one of his country's best hopes for a medal at this year's Summer Olympic Games in London.