The bus, carrying 52 passengers, crashed in a tunnel in Sierre in the canton of Valais on March 13th, 2012 while returning to Belgium from the Val d’Anniers skiing area.
Valais prosecutor Olivier Elsig said that after consulting experts he narrowed the likely cause of the accident to the two possibilities.
Elsig received two reports regarding the heart condition of the driver, as well as the antidepressant prescription drug he was taking.
Heart disease could have led to a malaise, later undetectable, which triggered the accident, the prosecutor said.
However, such incidents are rare, according to the expertise given to the prosecutor.
The experts also believed it was unlikely that the medication taken by the driver would have restricted his ability to drive properly.
Earlier evidence showed the driver had not been drinking at the time.
The bus drove into a concrete wall at the end of a turnout lane in the tunnel.
While most passengers aboard were Belgian citizens, six Dutch citizens were killed as well as a schoolboy with dual Belgian-British nationality.
Elsig said he now plans to close the investigation file on the accident.
A report of the prosecutor’s findings will be made available to interested parties who will have period in which they can seek further investigation into the incident, the ATS news agency reported.