Four Americans captured by Somali pirates while sailing in the Indian Ocean have been shot and killed, NBC News reported Tuesday.
The two couples, Phyllis Macay, 59, and Robert Riggle, 67, of Seattle, and the yacht's owners, Jean and Scott Adam of Marina del Rey, California, were on an around-the-world sailing trip when they were captured by pirates Friday.
Military officials told NBC News that about 1 a.m. ET shots were heard aboard the yacht, called Quest. Negotiations had been under way with the pirates at the time.
The officials said U.S. military personnel boarded the yacht and discovered all four hostages had been shot by their captors.
The officials said two pirates were killed and 13 others captured after a brief gun battle as U.S. forces took control of the boat.
"We express our deepest condolences for the innocent lives callously lost aboard the Quest," Gen. James N. Mattis, U.S. Central Command Commander, said in a statement.
Two pirates already dead
The statement added that in addition to the 15 dead or captured pirates, U.S. forces had found the remains of two pirates on the vessel who were already dead. It did not say how they had died.
The statement also said two pirates had been in U.S. custody before the Quest was boarded — as part of the negotiation process — and that in total it was believed 19 pirates had been involved. They are thought to have used a so-called mother ship to reach the Quest which was about 190 miles off the Oman coast.
The statement said U.S. forces had been "closely monitoring" the Quest for approximately three days with four Navy warships tasked to recover the yacht: the aircraft carrier USS Enterprise, the guided-missile cruiser USS Leyte Gulf, the guided-missile destroyers USS Sterett and USS Bulkeley.
Vice Adm. Mark Fox, Commander of the US Navy 5th fleet, told a news conference that negotiations were taking place between the U.S. forces and the pirates Tuesday morning when they suddenly fired a rocket-propelled grenade at the Sterett, which was about 600 yards away. The RPG missed.
Fox said gunfire was heard "almost immediately" afterward and then several pirates appeared on the deck of the yacht with their hands in the air, wanting to surrender.
A boarding party of U.S. special forces was sent across to the Quest but "despite immediate steps to provide life-saving care, all four American hostages died of their wounds," Fox said.
He said no shots were fired by the U.S. personnel as they boarded the yacht. However, as the yacht was being cleared later, the special forces shot dead one pirate and killed another with a knife.
Fox said they had not planned to launch a military operation against the yacht.
"The intent always had been this would be a negotiated process and not ever go to the point where we had gunfire," he said.
Asked about the two pirates who were already dead when the troops arrived, Fox said he did not know the circumstances of their deaths.
"We've seen a growing problem here in terms of pirate activity off the coast of Somalia," he said, with the mother ships allowing the pirates to strike as far away as the coast of India.
After the pirates captured the yacht off the coast of Oman, they headed for waters between Yemen and northern Somalia.
The yacht Quest which was hijacked by Somali pirates Friday.
Pirates vow revenge
Two Somali pirates who spoke with Reuters by telephone 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 northern semi-autonomous region of Puntland, 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.
Pirates have increased attacks on ships off the coast of East Africa, but Americans have rarely been targeted.
The last attack against a U.S. crew, which happened in 2009, ended with Navy sharpshooters killing two pirates and rescuing the ship's captain.
According to , the four Americans taken hostage after their yacht was hijacked by pirates off the coast of Oman in the  have been killed. Pirates hijacked their yacht, named ''The Quest'', on February 18. 

The U.S. Naval ship that was following the yacht, which was traveling off the coast of Somalia, heard gunshots and attempted a rescue operation. Naval personnel had been negotiating with pirates when the incident occurred. After boarding the vessel, soldiers killed 2 pirates and detained at least 13 others. During the process they discovered that the pirates had shot and killed all four hostages. 

"The forces discovered all four hostages had been shot by their captors. Despite immediate steps to provide life-saving care, all four hostages ultimately died of their wounds," said a statement issued by the U.S. Central Command.

The hostages were identified as Scott and Jean Adam, husband and wife who owned the yacht, from Marina Del Rey, California. Their two friends were Phyllis Macay and Bob Riggle from , Washington. Two other bodies were found on the yacht, believed to be pirates. The party was taking place in a race with other boats, which started in Thailand, when they decided to leave it after scheduled stop in Mumbai, India. Reports say they spent some of their time distributing bibles around the world. They changed course and were hijacked by the pirates off the coast of Oman.