The Newcastle star was interrogated by the Turin prosecutor's office for almost three hours and, according to La Gazzetta dello Sport, he informed them that he placed bets on football games, including matches involving Milan.
That admission complicates the situation. He insists he did not place bets on Milan matches when he was playing and, if this claim is accepted, Tonali will not be charged with match-fixing as he would not have influenced a match.
However, the fact he gambled on his own team could result in a ban longer than three years from the Italian Football Federation (FIGC) prosecutor.
Tonali has also declared that he suffers from a gambling addiction, which means part of his punishment could be diverted to mandatory therapy sessions.
The Italy international will hope to receive a similar punishment to Juventus midfielder Nicolo Fagioli, who was sentenced to a seven month ban plus a fine and therapy.
The FIGC prosecutor's office is not the only body looking into Tonali's bets, though, as there is also a criminal investigation into his activities on the illegal gambling platform.
The 23-year-old risks a substantial fine for using unauthorised betting websites. Investigators will be able to compare what he said in his statement with what they found on his mobile phone and tablet, which were confiscated.
As well as Tonali and Fagioli, Aston Villa's Nicolo Zaniolo has also been implicated in the betting scandal and faces a similar punishment.