Italy global and Serie A side Fiorentina's captain Davide Astori died at the age of 31 on Sunday, March 4.
"The belief is that the player died of a cardiac arrest by natural causes", Udine prosecutor Antonio De Nicolo said.
The magistrate added that it was "strange that such a thing happened without any warning symptoms in a profession which is so closely monitored".
Astori's body has been taken to the Santa Maria della Misericordia Hospital for an autopsy.
FIORENTINA captain and Italy worldwide Davide Astori has died suddenly in his sleep aged 31.
Fiorentina's game was called off in the morning.
Serie A postponed all of Sunday's matches including a derby between AC Milan and Inter Milan following the announcement.
Astori earned four of his 14 Italy caps under Conte between 2014 and 2016.
"Ciao dear Asto", Buffon wrote on Instagram.
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Buffon said he did not usually make a habit of expressing his feelings about others.
A visibly shaken Antonio Conte and World Cup victor Gianluigi Buffon paid emotional tributes to Davide Astori, who died at the age of 31 on Sunday, describing their Italian compatriot as "fantastic" and "a great guy".
"I'm shocked, it's an absurd tragedy", said Giovanni Malago, the president of Italy's national Olympic committee. He was a fantastic guy, a great player but a fantastic guy and I stay. I stay close to his wife, parents and daughter.
Chelsea coach Antonio Conte has paid tribute to his former player.
Unfortunately for the players and staff of Genoa and Cagliari the news was reported to them whilst both teams were warming up at the Marassi.
"How many battles we were in together at Cagliari, then found each other again at Roma I still can't believe it".
Astori came thought the ranks at AC Milan's Academy and while he never played for the Rossoneri in a senior game, his move to Cagliari helped establish himself in the Serie A, with many top clubs in the Italian top-flight taking notice of the 6'2 centre-back.
"A man who loved football and who grew up as footballer with us".
In a statement, Fiorentina lamented Astori's passing and wished his family well-wishes in this time of need. "Goodbye captain", said former Italian prime minister Matteo Renzi.