Check out CNN affiliate KING-TV in Seattle for a report on what was supposed to be a couple's dream trip.

(CNN) -- A round-the-world boating adventure ended tragically Tuesday for four Americans, whom pirates fatally shot after capturing their yacht in the Indian Ocean last week, U.S. officials said.

The 58-foot vessel, named the Quest, was being shadowed by the military after pirates took the ship off the coast of Oman on Friday. Officials had said earlier Tuesday it was less than two days from the Somali coast.

Ship owners Jean and Scott Adam and Phyllis Macay and Bob Riggle were found shot after U.S. forces boarded the vessel about 1 a.m. ET, officials said.

The forces responded after a rocket-propelled grenade was fired at a U.S. Navy ship about 600 yards away -- and missed -- and the sound of gunfire could be heard on board the Quest, U.S. Navy Vice Admiral Mark Fox told reporters.

"Despite immediate steps to provide life-saving care, all four hostages ultimately died of their wounds," U.S. Central Command said.

The incident took place as negotiations involving the FBI were under way for the hostages' release, Fox said. Two pirates had boarded a U.S. Navy ship Monday for the negotiations, he said. He told reporters he had no information on details of the negotiations or whether a ransom had been offered.

Two pirates were found dead on board the Quest, he said. In the process of clearing the vessel, U.S. forces killed two others, one with a knife, Fox said. Thirteen others were captured and detained along with the other two already on board the U.S. Navy ship. Nineteen pirates were involved altogether, he said.

The Adams were from Marina del Rey, California, Fox said, and Macay and Riggle were from Seattle.

The 15 detained pirates were being held together on a U.S. warship, Fox said, and "we will go through the appropriate process to bring them to a judicial process and hold them accountable for their activities."

Somali government officials condemned the killing.

"The Transitional Federal Government of Somalia deeply regrets the deaths," a government statement said. "We condemn the actions of these and all pirates operating from our shores and we will ensure that the captured pirates are brought to justice."

In Washington, the Justice Department issued a brief comment, with department spokesman Dean Boyd saying the United States "is committed to working with our international partners to ensure that the perpectrators of this heinous crime are brought to justice."

A second Justice Department official said there are a number of options for trial of the pirates. Two leading options would be to try the men either in the Southern District of New York or in the Eastern District of Virginia because prosecutors in those districts have experience handling such cases, the official said.

In New York, Abduwali Abukhadir Muse pleaded guilty to the 2009 hijacking of the container ship Maersk Alabama and for taking the ship's captain hostage. Last week Muse was sentenced to almost 34 years in prison for his crimes. He was the only surviving pirate involved in the take-over of the Maersk.

In Norfolk, Virginia, five Somali pirates were convicted after a nine day trial in November of attacking a Navy vessel, the USS Nicholas, which they had mistaken for a merchant ship. A second case in Norfolk involves alleged pirates accused of firing at another Navy ship, the USS Ashland.

The federal judge in the Ashland case threw out the lead count of piracy, but prosecutors are appealing that decision. The attacks on the Nicholas and the Ashland occurred last April.

Another option would be to bring the case in California or Washington, the home states of the victims. The Justice official said it would be possible to try the suspected pirates in almost any judicial district the Justice Department might choose by simply flying the suspects to that location and making it the suspects' first point of entry in the United States.

Fox said authorities believe the pirates were trying to get the vessel and hostages to Somalia, or at least into Somali territory waters.

Fox said it was the deadliest pirate hijacking involving U.S. citizens that he could recall. There have been fewer than 10 fatalities associated with pirate activity in the region in the past few years, he said.

The Adams, Macay and Riggle had been traveling with yachts participating in the Blue Water Rally since their departure from Phuket, Thailand, rally organizers said Sunday on the event's website. The group, which organizes long-distance group cruises, said the Quest broke off on February 15 after leaving Mumbai, India, to take a different route.

A statement from Blue Water Rallies on Tuesday called the four "brave adventurers." "We at Blue Water Rallies are stunned and devastated by the news of the loss of four friends who have had their innocent lives taken away from them by the pirate menace which is plaguing the Indian Ocean," it said.

The Somalis also issued condolences. "I do express a deep condolence to the families," said Omar Jamal, first secretary of the Somali mission to the United Nations, in a statement.

U.S. President Barack Obama was notified early Tuesday of the deaths, White House spokesman Jay Carney said. Obama had a briefing on the situation over the weekend and authorized the use of force against the pirates in the event of an imminent threat to the Americans' safety, he said.

The United States "strongly condemns the murder of four U.S. citizens," Secretary of State Hillary Clinton said in a statement, adding, "This deplorable act firmly underscores the need for continued international progress toward confronting the shared security challenge posed by piracy in the waters off the Horn of Africa."

Forces had been monitoring the Quest for three days, officials said. Four U.S. Navy warships were involved in the response force -- an aircraft carrier, a guided-missile cruiser and two guided-missile destroyers, according to the statement.

Fox said Tuesday authorities believe the 19 pirates came aboard the Quest after traveling on a "mother ship."

The "mother ship" trend -- pirates using another hijacked merchant vessel -- has appeared in the past few months, said Cyrus Mody, manager at the International Maritime Bureau in London. The mother ships provide pirates with "a lot more reach, a lot more capability to move out (farther) into the Indian Ocean," he said.

In addition, pirates can stay on board longer, have appropriate equipment and can demand the expertise of the ship's crew, he said. Previously, pirates typically hijacked a vessel and held it until a ransom was paid, he said.

The Adams were a passionate couple who spent most of their time since 2004 boating around the world, said Scott Stolnitz, who knew Scott Adam, a retired film executive, for nearly 40 years. The couple had a small boat at the Del Rey Yacht Club, where they would occasionally return to visit friends, family and handle business, he said.

But traveling the world on their yacht was where they really wanted to be, he said.

"They loved the experiences they were having with the people they were meeting and the places they were going," Stolnitz said. "We asked them once if they ever looked forward to living on land again, and they both, believe it or not, said no."

He earlier had said the Adams were conscious of pirate threats and were concerned about boating in the area.

One aspect of their travels, according to the couple's website, was "friendship evangelism -- that is, finding homes for thousands of Bibles, which have been donated through grants and gifts, as we travel from place to place." They also said their mission was to "allow the power of the Word to transform lives."

But, Stolnitz said, vigorous evangelism wasn't a major emphasis for the couple. "They use the Bible as an ice-breaker," he said.

Nina Crossland, Macay's niece, said she did not know if the Adams had any weapons aboard their yacht, nor did she know why they decided to break off from a sailing group that was traversing the dangerous waters of the Indian Ocean together.

"We don't know and now unfortunately we may never know," she asid.

Crossland said she was told that Macay was still alive when U.S. Navy SEALS arrived on board the Quest, but the sailors were not able to save her.

Asked earlier whether the U.S. forces were SEALS, Fox earlier said they were "U.S. special operations forces."

Piracy has flourished off Somalia, which has not had an effective government for two decades. While piracy in the Indian Ocean has taken place for years close to the Somali coast, "in 2008 we saw a very marked and rapid shift into the Gulf of Aden, where Somali pirates were attacking and hijacking vessels very, very regularly," said Mody of the International Maritime Bureau.

International navies combating piracy have been fairly successful in setting up a patrolled transit corridor through the Gulf of Aden, Mody and Fox said. But the pirates' activities then shifted into the southern Red Sea, Indian Ocean and the Arabian Sea area, Mody said.

In addition, there was "a very large increase" in pirate attacks between 2008 and 2009, although the increase did not continue into 2010, Mody said. He noted that in the first few months of 2010, virtually no pirate hijackings were reported. So far in 2011, "we have already seen more than 50 attacks carried out," he said. "From 2008, what we've seen is they have evolved ... and increased their capabilities."

International counterpiracy teams do what they can to control pirate activities in the region, he said, but "the area in which the Somali pirates are threatening is far too large for a concentrated naval effort like what is happening in the Gulf of Aden, so the navies are mostly relying on a lot of intelligence gathering and targeting vessels based on the intelligence."

The entire United States east of the Mississippi River could fit into the Somali basin, Fox said. "It's a vast, vast area." Currently, 34 warships patrol the region under 15 flags and work well together, he said, but "there's a lot of places where we are not."

In April 2009, pirates seized the U.S.-flagged Maersk Alabama, leading to a standoff in the Indian Ocean. U.S. forces moved to rescue American Captain Richard Phillips after seeing a pirate aiming a weapon at his back, officials said at the time.

Three pirates were killed and one was arrested. The Somali man arrested was convicted of acts related to high-seas piracy, and a federal court in New York sentenced him last week to more than 30 years in prison.

As of February 15, pirates were holding 33 vessels and 712 hostages, according to the International Maritime Bureau.

CNN's Mike Pearson, Ashley Hayes, Carol Cratty, David McKenzie and Jamie Crawford contributed to this report