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Four Americans aboard a yacht hijacked by Somali pirates were gunned down by their captors Tuesday.

U.S. forces responded to gunfire aboard the yacht Quest at approximately 1 a.m. Tuesday, but discovered all four hostages had been shot by their captors. Despite attempts to save their lives, all four hostages died of their wounds.

“We express our deepest condolences for the innocent lives callously lost aboard the Quest,” said Gen. James N. Mattis, U.S. Central Command Commander in a news release.

Two pirates died during the confrontation and U.S. forces found the remains of two other pirates already dead aboard the vessel. Thirteen pirates were captured and detained, along with two already in custody. A total of 19 pirates were involved in the hijacking.

The remains of the four Americans were taken aboard the aircraft carrier USS Enterprise, as were the 15 pirates in custody. There is no word yet on where the pirates will be taken for trial. They could go to Kenya or they could come back to the U.S. There is a precedent for both.

The yacht Quest was hijacked on Friday off the coast of Oman and U.S. forces had been closely monitoring the vessel.

Unlike most pirate incidents, these pirates boarded the Quest directly from their mother ship, rather than using faster skiffs. The mother ship remains free.

Vice Admiral Mark Fox, Commander of Centcom's Naval Forces, explained the timeline of events during a press briefing with Pentagon reporters. According to Fox, there was "absolutely no warning" before the hostage situation turned deadly.

On Monday two pirates boarded the USS Sterett (one of four U.S. Naval ships monitoring the situation) to conduct negotiations for the release of the American hostages. They stayed on board overnight and it's unclear if any ransom was offered before the killing took place.

At 8:00 am local time Tuesday morning a rocket propelled grenade was fired at the Sterett from pirates on board the Quest. The shot missed, but immediately after gunfire erupted inside the cabin of the Quest.

"Several pirates appeared on deck and moved up to the bow with their hands in the air in surrender," Fox said. That's when SOF approached on small boats and boarded the yacht.

When U.S. Special Operations Forces (SOF) soldiers -- from a classified Navy SEALS unit --  reached the yacht, they found two pirates had already been killed by small arms fire. As they went below deck there was an exchange of fire that killed one pirate. The other pirate was killed by an SOF member who used a knife in close combat, Vice Adm. Fox said.

The SOF found some of the Americans still alive, but all four soon died of their wounds. Vice Adm. Fox called it the deadliest pirate incident to date.

"We did everything we could," said a senior military official. "But I don't think our guys would see this as a good outcome."

Secretary of State Hillary Rodham Clinton called the killings "deplorable." She said in a statement that the killings underscored the need for international cooperation on fighting the scourge of piracy in waters off the Horn of Africa.

She urged international partners to provide material, financial and logistical support to an African peacekeeping mission in Somalia, the country the pirates use as the launching point of their attacks.

The organizers of an international yacht race called the Blue Water Rally said the Quest had been taking part in the race but left it Feb. 15 to chart an independent course from India to Oman.

The Quest is owned by Scott and Jean Adam, a couple from California. The two other Americans on board were Phyllis Macay and Bob Riggle, of Seattle, Washington.

At the Seattle Singles Yacht Club, where Riggle and Macay were well known, Joe Grande said the two were "great sailors, good people. They were doing what they wanted to do, but that's small comfort in the face of this."

The Adams have been sailing the world with a yacht full of Bibles since 2004. The hijacking of their yacht came two days after a Somali pirate was sentenced to 33 years in prison by a New York court for the 2009 hijacking of the Maersk Alabama. That case ended when Navy sharpshooters killed two pirates holding the ship's captain.

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. Multimillion-dollar ransoms are fueling the trade, and the prices for releasing a ship and hostages have risen sharply.

Pirates currently hold 30 ships and more than 660 hostages, not counting the attack against the Quest. Before this incident there were less than 10 pirate related deaths this year.

The best-known case of Westerners being held hostage in Somalia was that of Paul and Rachel Chandler, a British couple held for 388 days. The two, who were captured while sailing in their private yacht, were released in November.

The Adams -- who are members of the Marina del Rey Yacht Club in Marina del Rey, Calif. -- run a Bible ministry, according to their website, 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.

This is a developing story. Please refresh for additional updates.

The Associated Press contributed to this report.