Milan prosecutor Francesco Greco said an investigation had been opened following the death on March 1 of Moroccan-born Imane Fadil at one of the northern city’s hospitals.
The 34-year-old had been brought to the hospital on January 29 complaining of unexplained stomach pains and exhibiting “symptoms of poisoning”.
Ms Fadil’s death wasn’t announced until last Friday, more than two weeks after she died.
“The doctors have not identified with any certainty any pathology which can explain the death,” Greco said, adding there were “several anomalies” in her medical records.
According to Italian daily Corriera della Sera, toxicological exams run by a laboratory in Padua suggest the presence of “a mixture of radioactive substances which are not normally available for purchase”.
Ms Fadil had reportedly told her friends and lawyer she had been poisoned prior to her death.
Fadil was one of the main witnesses who testified at the 2012 trial of Berlusconi on charges of having sex with an underage prostitute at one of his infamous bunga-bunga parties.
She testified that the first time she went to a party, she saw two young women in nun costumes stripping in front of the then prime minister.
Later, she said the 82-year-old media mogul handed her €2,000 in cash, telling her: “Don’t be offended.”
Ms Fadil had told reporters she feared for her safety after telling prosecutors investigating possible witness tampering in the case that she was offered money in exchange for her silence regarding Berlusconi’s hedonistic parties.
Berlusconi is currently on trial for paying witnesses in Milan, Sienna, Rome and Turin to give false testimony about the parties.
