Initial tests on Imane Fadil, a 33-year-old Moroccan model who died in hospital in Milan, appear to have ruled out claims that she suffered from radioactive poisoning.
Investigators are trying to find out what led to the model's death, after prosecutors said there were "several anomalies" in her records.
Ms Fadil was a key witness in the trial of ex-Prime Minister Silvio Berlusconi.
She gave evidence in 2012 about his "bunga bunga" parties near Milan.
Mr Berlusconi, 82, was eventually cleared of paying for sex with an underage prostitute, but convicted of tax fraud.
He said at the weekend he had never met or talked to Ms Fadil and said "what I read of her statements made me think that everything was invented, absurd".
Why were the tests carried out?
Imane Fadil's death on 1 March emerged several weeks after she was admitted to the Humanitas research hospital on 29 January with stomach pains, saying she had been poisoned.
Prosecutors said she had died either from poisoning or a rare illness and opened a voluntary homicide investigation.
Reports said she had been writing a book about her experiences and that prosecutors had seized drafts of her work as well as her mobile phone.
They said unusually high levels of heavy metals had been found in her body - including cadmium, chromium and antimony. But Italian media also reported that radioactive substances such as cobalt had been involved.
Tissue samples from the model's kidney and liver were sent to specialists who said it was "increasingly unlikely" on the basis of initial tests that she had been contaminated by radioactive substances.
Further tests still have to be done at a research centre in Rome and a post-mortem examination will also be carried out.
Researchers now have to determine if she died of a rare illness that ravaged her immune system or if she suffered from metal poisoning.
Mr Berlusconi's reputation was tarnished by allegations about parties at his private villa at Arcore near Milan.
He is still facing a number of charges, including bribery. He denies the allegations.