Italian medical experts said more tests would be needed to find the exact cause of death of Livorno's Piermario Morosini following an autopsy on Monday.
The 25-year-old collapsed during an Italian second division game at Pescara on Saturday, prompting the country's soccer federation to postpone professional matches over the weekend.
Investigators have said they want to establish as much detail as possible as they are considering potential manslaughter charges.
Doctors tried to revive the former Italy under-21 international, who was at Udinese until January, but he was dead on arrival at hospital.
"There are no obvious signs that allow us to determine the boy's death," Christian D'Ovidio, a medical services official, was quoted by Italian news agency ANSA as saying after the autopsy, which lasted for most of Monday afternoon.
D'Ovidio said further examinations would be required, including toxicology tests.
Investigators are looking into whether an ambulance was blocked by the car of a police officer on its way to help Morosini.
The mayor of Pescara, Luigi Albore Mascia, said on Monday the police officer in question had accepted "full responsibility" in a written statement for parking his car in a spot that blocked the ambulance for a few minutes, and had volunteered his own suspension, Italian media reported.
Morosini, born in Bergamo and a former player with the city's Atalanta team, fell to the ground with no one near him in the 31st minute. He got up looking confused and then collapsed again as medics and then an ambulance rushed on to the field.
His funeral is scheduled to take place on Thursday.
Serie A side Udinese said on Monday it would care for Morosini's disabled sister, who relied on him for financial support since both their parents and other brother died.
Last month Bolton Wanderers midfielder Fabrice Muamba suffered a cardiac arrest during his side's FA Cup quarter-final against Tottenham Hotspur and needed 15 defibrillator shocks to restart his heart.
He has made a miraculous recovery and was discharged from a London hospital on Monday.
Photograph: Mike Cassese / Reuters