By Phil Stewart
WASHINGTON (Reuters) - Pirates shot dead four U.S. hostages on a private yacht on Tuesday, the deadliest incident involving Americans kidnapped for ransom in the increasingly dangerous waters off Somalia.
The U.S. military said the pirates shot the hostages before American special forces boarded the vessel.
U.S. troops killed two pirates as they took control of the boat, and took 15 pirates into custody. Another two pirates were found dead when the U.S. special forces arrived but they were not killed by U.S. forces, the military said.
Secretary of State Hillary Clinton said the U.S. government was "deeply saddened and very upset by the murder of four American citizens" -- a "deplorable act" that underscored the need for more international cooperation against the pirates.
"We've got to have a more effective approach to maintaining security on the seas, in the ocean lanes, that are so essential to commerce and travel," she told reporters.
Pirate gangs preying on shipping lanes through the Gulf of Aden and the Indian Ocean typically target large merchant ships, with oil tankers the prize catch, but the snatching of foreigners can also yield high ransoms. There were around 750 pirate hostages at the end of January.
The Americans killed on Tuesday were Jean and Scott Adam, from California, as well as Phyllis Macay and Bob Riggle from Seattle.
The U.S. military said negotiations with the pirates had been under way when on Tuesday morning, without warning, a pirate fired a rocket-propelled grenade at the guided-missile destroyer USS Sterett.
Then gunfire broke out inside the pirated vessel.
"The intent always had been that this would be a negotiated process and not ever go into a point where we actually had gunfire," said Vice Admiral Mark Fox, the head of U.S. naval forces in the turbulent region.
President Barack Obama had authorized the use of force in the case of an imminent threat to the hostages, White House press secretary Jay Carney told reporters.
In Seattle, people who knew Riggle, who was in his late 60s, said he was a retired veterinarian who had worked under contract for the Seattle Animal Shelter.
"He was a very nice, compassionate individual. He had a way with animals. He treated people with dignity and respect," said Dan Jordan, the director of the shelter.
Jordan said Riggle seemed well-suited to his companion, Macay. "He was a man of few words, and she was a woman of many words," he said.
In April 2009, U.S. Navy special forces freed the captain of the U.S.-flagged Maersk Alabama by killing three Somali pirates who held him hostage in a lifeboat. Obama had authorized the use of force in that incident as well.
The one surviving pirate from the Maersk incident has been sentenced to 33 years and nine months in prison in a federal court in New York.
Two Somali pirates who spoke with Reuters by telephone on Tuesday said the hostages were ordered killed since the pirates themselves were under attack by U.S. forces.
"Our colleagues called us this morning, that they were being attacked by a U.S. warship," Mohamud, a Somali pirate, told Reuters. "We ordered our comrades to kill the four Americans before they got killed."
Pirate leader Farah, speaking from Bayla, a pirate haven in the semi-autonomous region of Puntland in northern Somalia, vowed to avenge the deaths and capture of his comrades.
"I lost the money I invested and my comrades. No forgiveness for the Americans. Revenge. Our business will go on," he said, adding he had spent $110,000 so far in the hijacking, including on weapons and food and salaries.
(Additional reporting by Alister Bull in Washington and Mohamed Ahmed and Abdi Sheikh in Mogadishu; editing by Vicki Allen, Frances Kerry and Mohammad Zargham)