Divers descended some 50 meters (164 feet) below the sea surface on Tuesday to the wreck of a yacht off the coast of Sicily to search for six missing people.
While 15 people who had been aboard were rescued, one man was found dead. The other people on the vessel have not yet been located.
Among those feared dead were the chairman of London-based Morgan Stanley International, Jonathan Bloomer, and his wife, as well as British-Irish tech entrepreneur Mike Lynch and his daughter.
Lynch, a celebrated technology sector entrepreneur and investor, is sometimes referred to as a British answer to Bill Gates.
Chris Morvillo, a lawyer at Clifford Chance who represented Lynch in a US trial, was also among the missing, along with his wife.
Lynch was acquitted on all charges in a San Francisco court in early June, having been accused of an $11 billion fraud when his software firm Autonomy was sold to US tech giant Hewlett-Packard. Morvillo was Lynch's defense attorney in the case.
It is understood that Lynch had invited friends to stay aboard the yacht to celebrate his legal victory, at which Bloomer was a defense witness.
In an uncanny coincidence, Lynch's co-defendant in the trial Steve Chamberlain died after he was hit by a car while out running on Saturday.
Rescue teams on Monday retrieved the body of the British-flagged vessel's onboard chef, identified as Antiguan citizen Ricardo Thomas.
Divers trained to explore tight spaces were flown to Sicily from Rome and Sardinia late Monday, but an initial nighttime search of the wreck failed.
"Access was limited only to the bridge, with difficulty due to the presence of furniture obstructing passage," the fire service said on X.
Nine other crew members were among those rescued, as well as Lynch's wife and a mother with her one-year-old baby.