Officials say Christopher and Neda Morvillo are still missing on Tuesday.
Italian officials say it happened just before dawn when a possible waterspout hit the area.
Twenty-two people were onboard at the time. Fifteen made it into a life raft - including a mother and her 1-year-old baby.
Morvillo is a former U.S. attorney who worked on the 9/11 criminal investigation and now works in private practice for the global law firm Clifford Chance.
"We are in shock and deeply saddened by this tragic incident. Our thoughts are with our Partner, Christopher Morvillo, and his wife Neda who are among the missing. Our utmost priority is providing support to the family," the firm said in a statement.
Morvillo was aboard the yacht owned by British tech CEO Mike Lynch as they celebrated a victory in a 10-year-long multi-billion dollar-fraud trial.
Lynch and his 18-year-old daughter Hannah are also missing, along with Morvillo and his wife.
The bodies of the missing victims may be trapped inside the vessel which has been difficult to reach by rescuers.
Some of the 15 people who were rescued are either still recovering or have now left hospital.
RELATED | British tech magnate Mike Lynch, 5 others missing after luxury superyacht sinks off Sicily
British tech magnate Mike Lynch, 5 others missing after luxury superyacht sinks off Sicily
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