Four Somali pirates also died, two of them killed by US special forces in one of the deadliest endings to a raft of hostage-takings off the coast of Somalia which often are resolved through ransom payments.
Jean and Scott Adam, a California couple active in missionary work, had been sailing the world on the S/V Quest yacht for more than seven years and had planned to take in sites from India to Djibouti to Crete on their latest trip.
Phyllis Macay and Bob Riggle, a couple from Seattle who joined the Adams, were also killed by the band of 19 pirates who commandeered the yacht in waters southeast of Oman, US officials said.
“We express our deepest condolences for the innocent lives callously lost aboard the Quest,” General James N Mattis, the head of US Central Command, said in a statement.
President Barack Obama had authorised the use of force in case of an imminent threat to the hostages and was informed of the “tragic outcome,” his spokesman Jay Carney said.
Four US warships including the nuclear-powered USS Enterprise aircraft carrier had been tracking the S/V Quest since a Danish crew discovered three days earlier that it had been hijacked, US officials said.
The US military brought two of the pirates onboard the USS Sterett on Monday to conduct negotiations to free the hostages, said Vice Admiral Mark Fox, head of the US Naval Forces Central Command based in Bahrain.
Then yesterday morning, with “absolutely no warning,” the pirates launched a rocket-propelled grenade at the USS Sterett, Fox said, leading US Special Forces to race to the yacht where they heard gunfire.
By the time they boarded, all four Americans were dead from gunshot wounds. The US Special Forces took control of the yacht, stabbing to death one pirate and shooting dead another, Fox told reporters in Washington by telephone.
Two more Somali pirates were found dead inside the yacht in unclear circumstances, possibly the result of an earlier fight among the hijackers or of the bullets that killed the hostages, US officials said.
The Adams had been active in missionary work. On a previous yacht tour of islands across the South Pacific, the couple stopped off to distribute Bibles, according to their website.
The couple had kept a blog on their trip which began in early January in the Thai resort of Phuket. Jean Adam voiced hope at seeing the Taj Mahal, even though their docking point in India in the southern state of Kerala is far away from the world-famous monument to love.
Jean Adam described Djibouti, past Somalia and the Gulf of Aden, as “a big refuelling stop”.
“I have no idea what will happen in these ports, but perhaps we’ll do some local touring,” she wrote.
The 15 surviving pirates from the clash are being held on a US ship, Fox said.