NAIROBI, Kenya -- The U.S. Navy destroyer had been shadowing a hijacked yacht with four Americans aboard for days, in radio contact with their captors in waters off East Africa. Two pirates had even boarded the warship to negotiate.
Then without warning Tuesday, the situation turned deadly. From the yacht came a rocket-propelled grenade, followed by the staccato sound of gunfire. U.S. special forces scrambled onto the occupied vessel to find the Americans fatally wounded.
The yachting enthusiasts from California and Washington state were the first Americans slain by Somali pirates since a wave of attacks began six years ago. One of the American couples had been sailing around the world since 2004 handing out Bibles.
The deaths of the four travelers, all in their late 50s or 60s, appeared to underscore an increasingly brutal and aggressive shift by pirates in their treatment of hostages.
Killing hostages "has now become part of our rules," said a pirate who identified himself as Muse Abdi. He referred as a turning point to last week's sentencing of a pirate to 33 years in prison for the 2009 attack on the U.S. cargo vessel Maersk Alabama.
"From now on, anyone who tries to rescue the hostages in our hands will only collect dead bodies," Abdi said. "It will never, ever happen that hostages are rescued and we are hauled to prison."
Secretary of State Hillary Clinton condemned the killing of the Americans as "deplorable," saying in a statement that the slayings underscored the need for international cooperation in fighting the scourge of piracy in waters off the Horn of Africa.
Jean and Scott Adam of Marina del Rey, near Los Angeles, had made their home aboard their 58-foot yacht Quest since December 2004, and recently had been joined by Phyllis Macay and Bob Riggle of Seattle.
Four U.S. warships had been shadowing the Quest since shortly after it was seized south of Oman on Friday, and U.S. officials were in radio contact with the captors as the pirates tried to sail it to the Somali shore.
The power behind such abductions for ransom -- a multimillion-dollar business -- lies not with the pirates at sea but with their financial backers on land. And once the kidnappers reach shore with their hostages, options for rescue are limited.
A channel of negotiations had been opened, and on Monday two pirates boarded the USS Sterett, a guided-missile destroyer some 600 yards from the seized yacht, and they stayed overnight, said Vice Adm. Mark Fox, commander of the U.S. 5th Fleet in Bahrain.
By the next morning things quickly turned deadly, with all signs pointing to a dispute among the pirates.
At 8 a.m. local time, Fox said, a rocket-propelled grenade was fired from the Quest at the Sterett and missed, followed almost immediately by the sound of small-arms fire coming from the yacht.
Several pirates then appeared on the yacht deck with their hands up. U.S. naval forces rushed aboard the vessel and found all four Americans had been shot; two pirates also lay dead from gunshot wounds.
The special forces troops tried to provide lifesaving care to the Americans, but they died, Fox said.
Fifteen pirates were taken into custody, 13 aboard the yacht and the two who had been negotiating aboard the Sterett, Fox said. In addition, two pirates were killed in the operation, including one who was knifed by a member of the U.S. force, Fox said.
President Barack Obama, who was notified about the killing of the Americans at 4:42 a.m. Washington time, had authorized the military on Saturday to use force in case of an imminent threat to the hostages, said White House press secretary Jay Carney.
Pirates have increased attacks off the coast of East Africa despite an international flotilla of warships dedicated to protecting vessels and stopping the pirate assaults.
The conventional wisdom in the shipping industry had been that Somali pirates are businessmen looking for a multimillion-dollar ransom payday, not insurgents looking to terrorize people.
"We have heard threats against the lives of Americans before, but it strikes me as being very, very unusual why they would kill hostages outright," said Graeme Gibbon-Brooks, the head of Dryad Maritime Intelligence, adding that the pirates must realize that killing Americans would invite a military response.
Friends, family and fellow sailors said that despite an adventurous spirit, the four Americans were meticulous planners who knew the dangers they faced.
The Adams, both in their 60s, wrote on their blog of "trying to find homes for thousands of Bibles" during their travels to remote regions. They were joined by Riggle, a veterinarian who volunteered at the Seattle Animal Shelter, and Macay, a sailing enthusiast.
"Great sailors, good people. They were doing what they wanted to do, but that's small comfort in the face of this," said Joe Grande of the Seattle Singles Yacht Club, where Riggle and Macay were members.
The Adams had traveled from Panama in 2005 to Fiji in 2007 and Cambodia last year. They most recently sailed from Thailand to Sri Lanka and India, and were on their way to Oman when captured.
Pirates have become increasingly bold in their attacks despite a flotilla of international warships patrolling the waters off East Africa. The last time pirates kidnapped a U.S. citizen, during the 2009 hijacking of the Maersk Alabama, Navy sharpshooters killed two pirates and rescued the cargo ship's captain.
But Tuesday's bloody events are apt to leave U.S. military planners in a quandary. Do they go after the pirates harder? Do they attack their bases on Somalia's ungoverned shores?
One maritime expert said it's too early to tell.
"This is a first," said Gibbon-Brooks, the analyst. "We don't know if the situation is related to a straight execution. We don't know if it was related to an attempt to break free. We don't know if it was related to an accident."