She had reportedly been telling friends recently that she had “many things to reveal.”
A lawyer who represents two other young women who gave evidence against Mr Berlusconi, Ambra Battilana and Chiara Danese, called the death “strange and extraordinary”.
Mauro Rufini said prosecutors needed to get to the bottom of it as quickly as possible.
Now 82, the billionaire businessman is currently on trial for paying a witness to give false testimony about his parties.
Mr Berlusconi is already being investigated or prosecuted for witness tampering in Milan, Siena, Rome and Turin, each time for allegedly paying people to keep quiet about his bunga bunga parties.
He said he was saddened by Ms Fadil’s death but claimed he never knew the model. “It’s always very sad when someone as young as this dies,” he said. “I never met this person and never spoke to her.”
He said he was aware that she had claimed his gatherings were sex parties, but said such allegations were “invented and absurd.”