Imane Fadil
Model Imane Fadil arrives at Milan's court during the trial of the ex-premier Silvio Berlusconi for allegedly having sex with an underage prostitute on April 16, 2012. OLIVIER MORIN/AFP/Getty Images

Italian authorities on Friday announced an investigation into the death of a Moroccan model who had served as a key witness against former Italian Prime Minister Silvio Berlusconi in a 2012 underage sex trial.

Imane Fadil, 34, who was once a frequent guest at Berlusconi's "bunga bunga" parties, was admitted to a Milan hospital on Jan. 29 after complaining of severe stomach pains. She told friends and her attorney at the time that she had been poisoned.

Reports surfaced Friday of Fadil's death on March 1. There has yet to be a confirmed caused of death.

"The doctors have not identified with any certainty any pathology which can explain the death," Milan chief prosecutor Francesco Greco told Reuters. He said there were "several anomalies" in Fadil’s medical records.

Italian news agency ANSA widely reported that Fadil was in fact fatally poisoned by radioactive material, while newspaper Corriere della Sera reported that samples taken at the Milan hospital were sent to an outside laboratory for specialized testing.

Berlusconi was charged in 2012 with paying an underage woman for sex after attempting to cover up the incident by using his political influence. It was discovered that Berlusconi had called a police station in 2010 to request the release of a 17-year-old girl named Karima "Ruby" El Mahroug, who was being held by authorities for theft, the same girl he had allegedly paid for sex.

Along with two other witnesses who were frequent guests at the "bunga bunga" sex parties, Fadil testified against Berlusconi in 2012.

Berlusconi in 2013 was found guilty and sentenced to seven years in prison for paying El Mahroug for sex and abuse of power but the conviction was overturned in 2014 after it was ruled that he could not have known El Mahroug was underage.

Fadil had expressed disappointment over the acquittal.

"From seven years to zero – how is that possible?" she told La Repubblica newspaper at the time.

"In Milan, in certain circles, it was often discussed that by going to bed with the prime minister one could earn money," she added.

Berlusconi has since been charged with bribing former members of his parties to lie on the stand during his initial trial.

The Associated Press reported that Fadil had aspirations of becoming a sportscaster. She also reportedly was writing a book about her experiences as a "bunga bunga" guest and her life during and after the trial. Investigators have seized the untitled manuscript.

Fadil was also part of an on-going class-action civil suit against Berlusconi, arguing over lost opportunities due to her involvement in past cases.

Berlusconi on Saturday expressed condolences for Fadil's death, while denying he had ever met her and refuting her 2012 testimony.

"It's always a pity when a young person dies. I never met this person, never talked to her," Berlusconi said.

"What I read of her statements made me think that everything was invented, absurd."