Six hours of autopsy on Monday failed to produce an answer as to what caused the death of Livorno footballer Piermario Morosini, who collapsed and died during a match on Saturday.
Dr Cristian D'Ovidio said following the autopsy that more tests would be needed to find an exact cause of death.
"There is no evidence to the naked eye that allows us to determine what caused this lad's death," he said.
"Now we need to make more tests, including a toxicology report."
Morosini collapsed during Saturday's Serie B match at Pescara before being rushed to hospital, where he was later pronounced dead.
His heart had stopped beating on the pitch and doctors failed to resuscitate him.