The Italian news agency Ansa reported toxicology tests found the model had been killed by a "mix of radioactive substances". He was later convicted of tax fraud and sentenced to carry out community service.
Ms Fadil was writing a book about her experiences at the sex parties and the manuscript was picked up by Italian authorities after her death, local media reports.
Mr Berlusconi has always maintained he is "no saint" but firmly rejects claims he has ever paid for sex with a woman, saying: "I never understood where the satisfaction is when you're missing the pleasure of conquest".
"What I read of her statements made me think that everything was invented, absurd", he said.
Ms Fidal testified in court against Berlusconi - who is accused of bribing models, actresses and escorts to lie about the sex parties often hosted at his home.
"The doctors have not identified with any certainty any pathology which can explain the death".
Italy's Corriere della Sera newspaper reported that Milan's hospital had sent samples to a specialist laboratory.
This has not been confirmed by Italian prosecutors.
In 2012, Fadil had told reporters that 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 about what went on at Berlusconi's parties.
Mr Berlusconi's reputation was tarnished by allegations about the parties at his private villa.
Back in May 2010, a 17-year-old exotic nightclub dancer, Karima "Ruby" El Mahroug, left police custody in Milan after Prime Minister Silvio Berlusconi called surprised officials to say that the teen should be released because she was the granddaughter of the Egyptian president.
Berlusconi admitted that young women attended his soirees, but said the parties were nothing more than "burlesque contests".