Official reports had said Ozal died of a heart attack, which his private doctor said was the cause, Today's Zaman reported.
But the State Audit Institution said in a report Ozal was not suffering any chronic disease and noted no autopsy was performed after his death, which the report called an "eclipse of reason."
Turkey's President Abdullah Gul ordered the SAI investigation, which began in March.
"The sudden death of any president on duty is a suspicious one," the report stated.
Ozal's wife, children and close friends have suggested he might have been poisoned.