Scott and Jean Adam first set sail on the 58-foot craft in 2002 Four American sailors hijacked by Somali pirates off the coast of Oman have been killed, US defence officials say.
The US military said its forces trailing the vessel had responded to gunfire heard aboard but found all the captives shot when they arrived.
The yacht S/V Quest, hijacked on Friday, was owned and sailed by Scott and Jean Adam of California.
Also killed were two US passengers, Phyllis Mackay and Bob Riggle.
In a statement, US Central Command said that negotiations were underway between the US Navy and the pirates, when the US forces heard gunfire coming from the Quest about 0600GMT.
They boarded the ship, killing two pirates in the process, and discovered the four Americans shot. The US Navy sailors attempted to provide first aid but the hostages died, the military said.
“As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors,” Gen James Mattis of US Central Command Commander said in a statement.
“We express our deepest condolences for the innocent lives callously lost aboard the Quest,” the statement added.
The US Navy captured 13 pirates, and found the remains of two other pirates already dead about the vessel, the US military said.
The US Navy said it had been closely monitoring the vessel once it learned it had been hijacked, sending four warships to the area.
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