Prosecutors in Italy are investigating the captain of a superyacht that sank during a storm off Sicily last week, killing seven people.

Italian media reports that 51-year-old New Zealander James Cutfield, is being investigated for possible manslaughter and culpable shipwreck.

Under Italian laws, being under investigation does not imply being guilty and does not necessarily lead to criminal charges.

Prosecutors in Termini Imerese did not reply to calls to confirm the reports.

On Saturday, chief prosecutor Ambrogio Cartosio confirmed an investigation had been launched into last week’s deadly sinking of the luxury superyacht Bayesian off Sicily but said a suspect had not been identified.

He said the yacht had already sunk by the time rescuers arrived after last Monday’s storm.

He described the incident as “grave tragedy”. To “reduce the dimensions of the tragedy”, a “big operation of firefighters and firefighting divers who have shown incredible courage and skill” was called.

Cartosio said they had carried out a very difficult mission to find those missing in the sunken yacht.

The local fire chief said coastguard rescuers took 123 dives during the operation, staying a maximum of only 12 minutes in the water each time.

Cartosio said his team would consider each possible element of responsibility for the tragedy, including those of the captain, the crew, individuals in charge of supervision and the yacht’s manufacturer.

Cutfield was among 15 survivors of the sinking that killed British tech magnate Mike Lynch, his daughter Hannah and five others.

He told Italian media last week that the speed of the water spout that hit off Sicily made a death trap out of the superyacht.

“We just didn’t see it coming,” he said.

The Bayesian, a 56-metre luxury yacht, went down near the Mediterranean island in southern Italy last Monday.

Investigators are focusing on how a sailing vessel deemed “unsinkable” by its manufacturer, Italian shipyard Perini Navi, sank while another boat nearby remained largely unscathed.

Prosecutors said the event was “extremely rapid” and could have been a “downburst” – a localised, powerful wind that descends from a thunderstorm and spreads out rapidly upon hitting the ground.

The crew was saved, except for the chef. Six passengers were trapped in the hull.

Under maritime law, a captain has full responsibility for a ship and its crew, as well as the safety of all aboard.

-with AAP