A banner outside Fiorentina's stadium reads 'Bye Captain'
Corriere dello Sport and Tuttosport reported that doctors who carried out the player's autopsy referred to a “cardiac arrest without a visible explanation, in all likelihood due to a bradyarrhythmia [slow heart rate]”.
The true extent of the diagnosis is not set to be discovered for another couple of months.
Astori had made 14 appearances for Italy since making his international debut in 2011 and had been with Fiorentina since 2016.
He was found in his hotel room by a club masseuse after failing to turn up for breakfast on Sunday.
Davide Astori: A look at the Fiorentina captain's incredible defending
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There was a moment silence at Wembley ahead of Tottenham's clash with Juventus
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Udine prosecutor Antonio de Nicolo said: “We won't have the definitive diagnosis for two months [and then] we will be able to say with certainty that there was no detectable cause or any other element of any type.”
Fiorentina players and staff paid tribute to their captain at the club’s Artemio Franchi stadium earlier this week.
Scarves, banners and flowers adorn fences outside the ground while the players hung up two signed Astori jerseys as well as a large photo of the defender.
Astori’s funeral is set to be held today (Thursday) before his burial near Bergamo, in northern Italy, where Astori grew up and his parents still live.
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Scarves and flowers were laid outside Fiorentina's stadium
Juventus defender, and Astori’s national team mate Giorgio Chiellini was one of many to pay tribute to the defender.
Following Juve’s Champions League win over Tottenham, Chiellini told BT Sport: “We dedicate him not only this match but every mind in this day. I have cried many times, because he was fantastic player, and it is difficult because we have to think about the game.
“He is always in our heart, and tomorrow we go to give the last ciao together with our team mates.
“It is difficult, but we have to continue with our lives. It is important with the national team, in our heart, to always bring his positive emotion and his smile. We would like to continue his smile in our heart."