The defender, 31, was found dead in his hotel room on Sunday morning.
Initial reports were that he had suffered a heart attack but criminal proceedings have been opened following allegations of manslaughter, reports Gazzetta dello Sport.
“We have opened criminal proceedings over the hypothesis of manslaughter," Antonio de Nicolo, Udine chief prosecutor, told Radio Rai.
“For now, it's been brought against unknown persons. We've arranged an autopsy, which will be performed soon."
An autopsy is taking place on Astori's body on Monday to confirm the cause of death.