NAIROBI, Kenya -- A U.S. Navy destroyer was shadowing a hijacked yacht with four Americans aboard when a pirate fired a rocket-propelled grenade, followed by bursts of gunfire. U.S. special forces scrambled onto the occupied vessel only to find the four Americans fatally wounded.

The yachting enthusiasts from California and Washington killed off the coast of East Africa on Tuesday 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 the Maersk Alabama -- just two days before the hijacking.

"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 Rodham Clinton strongly condemned the killing of the Americans as "deplorable," saying in a statement 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 been sailing their 58-foot yacht Quest around the world since December 2004, and had been joined in recent months 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 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, though, 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 gun shot 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 as well as 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.

Pirates -- who currently hold 30 ships and more than 660 hostages -- typically win a multimillion ransom for releasing their captives, a huge sum that is shared among investors and pirates. The money is often spent on alcohol, drugs and prostitutes. One ransom paid last year was reported as $9.5 million. Most ransoms are worth several million dollars.

Given that typical financial motivation, Tuesday's killings left several unanswered questions, such as whether the four hostages had tried to take over the yacht from the pirates, or if the American forces spooked the pirates by approaching the yacht.

Pirates have increased attacks off the coast of East Africa in recent years despite an international flotilla of warships dedicated to protecting vessels and stopping the pirate assaults.

The Adams ran a Bible ministry and have been distributing Bibles to schools and churches in remote villages in areas including the Fiji Islands, Alaska, New Zealand, Central America and French Polynesia.