France team doctor Jean-Marcel Ferret has expressed surprise that Cameroon midfielder Marc-Vivien Foe's death was caused by a heart-related condition.

"Foe passed a thorough medical examination when he joined Olympique Lyon in 2000," Ferret, who was then team doctor at the French Ligue 1 club, was quoted as saying by the French daily Le Parisien on Wednesday.

"He was declared absolutely fit to play soccer. He was not suffering from any heart malformation," he added.

The autopsy on the 28-year-old player, who collapsed during Cameroon's semi-final win over Colombia in the Confederations Cup on June 26 in Lyon, found that he died from a "hypertrophic cardio myopathy".

"The death is from natural causes...It's the hyper development of the left ventricle," said Lyon prosecutor Xavier Richaud when he announced the post mortem result on Monday.

The magistrate added that Foe's heart condition was probably congenital and "almost untraceable without carrying out an extensive examination".

Ferret said "the 2000 medical tests were carried out by one of the best heart specialists in Lyon, doctor Jean Boutarin, head of the cardiology department at Lyon Saint-Joseph hospital".

"Dr Boutarin checked his files and I can tell you he was very surprised by the results of the autopsy."

Foe's post mortem was conducted by an International Olympic Committee-sanctioned Swiss laboratory in Geneva.

Richaud told Monday's news conference that "no stimulant substances were found".

"As far as blood and urine are concerned, the negative toxicology shows that he did not take any stimulants...in the 72 hours before his death. An analysis of his hair shows that he did not take stimulants before or regularly," he said.

Ferret told Le Parisien he was not hinting that experts and soccer officials were trying to cover up a doping case.

"With my cardiologist friends we are going to carry out our own investigation," he said.

Foe was buried with full national honours in Cameroon's capital Yaounde on Monday.