Searches resumed Thursday for the last person missing after a super yacht carrying UK tech tycoon Mike Lynch sank off Sicily, with divers recovering a fifth body from the wreck.

Specialist divers working with an underwater robot on Wednesday pulled up four bodies from the wreck of the "Bayesian", while another was pulled up on Thursday morning, according to AFP reporters.

The grim discovery brings the death toll so far to six, after the body of a man believed to be the yacht's chef was found shortly after the ship went down in a storm before dawn on Monday.

There was no official identification of the bodies, but Lynch and his 18-year-old daughter Hannah were among the six passengers missing.

The 56-metre (185 feet) British-flagged sailing boat had been anchored some 700 metres off Porticello, near Palermo on the north of the Italian island, when it was struck by a waterspout -- akin to a mini-tornado.

It sank within minutes.

Fifteen people were rescued, including Lynch's wife and a woman with a one-year-old baby.

Lynch and his daughter, his lawyer Christopher Morvillo and his wife Neda, and Jonathan Bloomer, the chair of Morgan Stanley International, and his wife Judy, were all reported missing.

Many questions remain about why the yacht sank, and on Thursday the head of the company which built the boat said the tragedy could have been avoided.

"Everything that was done reveals a very long summation of errors," said Giovanni Costantino, head of the Italian Sea Group, which includes the Perini Navi company that built "Bayesian".

He told Italy's Corriere della Sera newspaper that bad weather was forecast and all the passengers should have been gathered at the assembly point, all the doors and hatches closed.

Security camera footage of the ship from the shore showed the lights on its mast going out, which Costantino said indicated a short circuit, meaning that the ship had already taken on water.

"A Perini ship resisted Hurricane Katrina, a category 5 (hurricane). Does it seem to you that it can't resist a tornado from here?" he told the newspaper.

"It is good practice when the ship is at anchor to have a guard on the bridge, and if there was one he could not have failed to see the storm coming.

"Instead it took on water with the guests still in the cabin... They ended up in a trap, those poor people ended up like mice."