UK tech tycoon Mike Lynch, his 18-year-old daughter Hannah, four friends and the yacht's cook died when the British-flagged Bayesian sank in a storm before dawn on Monday.
"The Public Prosecutor's Office of Termini Imerese has registered a file with the state against unknown persons, hypothesising the crimes of negligent shipwreck and multiple negligent manslaughter," state prosecutor Ambrogio Cartosio told reporters on the Italian island.
But he said he was only announcing the probe due to the huge international interest in the case, stressing: "We are only in the initial phase of the investigations.
"At this stage, precisely because the investigation could develop in any way, we are absolutely not ruling anything out."
Lynch, 59, had invited friends and family onto the boat to celebrate his recent acquittal in a massive US fraud case.
But the 56-metre (185-foot) yacht was struck by something akin to a mini-tornado before dawn on Monday as it was anchored off Porticello, near Palermo.
Fifteen people were rescued and the body of the yacht's chef was found shortly afterwards. Six passengers were reported missing.
A major search operation including specialist divers subsequently identified the bodies of four of Lynch's friends on Wednesday.
Lynch's body was found on Thursday and that of his daughter, who had been preparing to go to Oxford University, on Friday.
Scrabbling for air
The bodies were found in two cabins on the side of the ship closest to the surface, and officials said they believed the trapped passengers had moved there in a bid to find pockets of air.
The ship sank by its stern -- the back -- and came to rest on its right side on the sea bed, some 50 metres down, Girolamo Bentivoglio Fiandra of the fire service said.
He told reporters the passengers inside "took refuge to seek safety in the cabins on the left side where somehow the last air bubbles formed".
"We found the first five bodies in the first cabin on the left side, and the last body in the third cabin on the left side," he said.
Bayesian's emergency flare had gone up at 4.38 am (0238 GMT) on Monday morning, and the coastguard immediately deployed -- but when they got there, it had already sunk.
Sudden, unexpected
Contrary to previous reports, Raffaele Macauda, from the Sicilian coastguard, said there had been no storm warning that night.
"It was a sudden and unexpected event," added Raffaele Cammarano, another state prosecutor.
The officials said the crew were free to leave Sicily during the investigation.
Information on whether the yacht's doors were open -- which might explain why it sank so fast -- would not be possible to confirm until the wreck was recovered, the officials said.
This could take weeks.
Lynch, once dubbed the British "Bill Gates", founded software firm Autonomy in the 1990s. Its $11 billion sale to Hewlett-Packard in 2011 saw him face fraud charges in the United States.
A jury in San Francisco acquitted the 59-year-old and a co-defendant of all charges in June.
After Hannah's body was brought ashore on Friday, her family issued a statement describing their "unspeakable grief".
"The Lynch family is devastated, in shock and is being comforted and supported by family and friends," it said.
US lawyer Chris Morvillo, a partner at top law firm Clifford Chance who worked on the US trial, also died along with his wife Neda.
The bodies of Jonathan Bloomer, the chair of Morgan Stanley International, and his wife Judy, were also found.
The chef was named in various media reports as Canadian-Antiguan Recaldo Thomas.
