WASHINGTON — A California couple and two other Americans taken hostage by Somali pirates were mortally wounded Tuesday morning by their captors, shortly before a U.S. special operations team boarded the hijacked vessel, killed two of the pirates and captured the rest, U.S. military officials said.

Some of the victims were still alive when they were found by the U.S. team and were given first aid, but all four died, said Vice Adm. Mark Fox, the commander of U.S. naval forces in the region.

The owners of the yacht Quest, Jean and Scott Adam of Marina del Rey, Calif., and another couple, Phyllis Macay and Robert Riggle of Seattle, were on an around-the-world sailing trip when they were taken hostage by pirates Friday off of Oman.

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

Fox told reporters at the Pentagon on Tuesday that the incident was the deadliest one he could recall involving U.S. citizens held by pirates.

A flotilla of U.S. naval vessels had been shadowing the yacht, known as the Quest, for three days and conducted negotiations over the weekend in an effort to free the two couples as the yacht made its way south toward Somalia, said Lt. Col. Mike Lawhorn, a spokesman for U.S. Central Command, which oversees U.S. anti-piracy operations in the Indian Ocean.

There were signs of divisions among the 19 pirates during the hostage standoff, U.S. officers said. On Monday, two of them came aboard one of the Navy vessels, the USS Sterett, for face-to-face negotiations and did not return to the yacht.

The incident turned fatal Tuesday morning when the pirates fired a rocket-propelled grenade at the Sterett, which missed, and U.S. naval personnel heard gunshots coming from the yacht. At that point, a team of 15 special-operations forces boarded the yacht.

“”As they responded to the gunfire, reaching and boarding the Quest, the forces discovered all four hostages had been shot by their captors,”” according to a U.S. military account of the incident.

Fox said he had no details of the negotiations with the pirates and declined to comment when asked if the U.S. had planned to prevent the hostages from being taken ashore if the yacht reached Somalia.

After the grenade was fired at the Sterett, several pirates came on deck with their hands raised, as if trying to surrender, Fox said. The gunfire erupted on board almost immediately. But U.S. officers said it was not known whether the hostages had made an escape attempt or whether disagreements among the pirates prompted the shots.

“”I can presume inside the vessel there was a lot of small-arms fire,”” Fox said, but he noted that the special forces team did not have to fight its way onto the yacht.

As the U.S. special forces team cleared the vessel, it discovered two pirates who already were dead. Another two were killed by U.S. personnel, one by gunfire and one by a knife, Fox said.

“”A pirate was killed by a special operations force members with a knife while clearing the interior of the vessel,”” Fox said.

In all, 15 pirates are in U.S. custody and will be held for possible prosecution, Fox said.

The FBI is investigating the deaths, Fox said.

Fox said there were indications that the pirates who took the Quest came from a “”mother ship,”” a larger vessel that operates far from Somalia and can dispatch teams to hijack multiple ships.

U.S. officials have said recently that as the U.S.-led effort to guard ships against Somali attacks had begun reducing attacks near the Somali coast, the pirates have extended their operations hundreds of miles out into the Indian Ocean to elude multinational naval forces, which includes dozens of vessels from two dozen countries.

Along with the Sterett, a guided-missile destroyer, the U.S. naval ships involved in the operation included the aircraft carrier USS Enterprise, the guided-missile cruiser USS Leyte Gulf and the USS Bulkeley, another guided-missile destroyer, according to Central Command.

They had begun tracking the yacht after being alerted that a Danish naval helicopter had seen the Quest off Oman under the pirates’ control.

Jean and Scott Adam of southern California owned the 58-foot custom-made yacht and had spent most of the last 10 years on sailing adventures to far-flung locales such as the Galapagos Islands, Tahiti and New Zealand. Posting photos and information on their website, they raved about their travels aboard the Quest. “”We’ve decided to … explore Fiji like petals on a flower,”” they wrote about their 2007 trip to the South Pacific.

On the trip in which they were hijacked, the Adams planned to travel across the Indian Ocean from their temporary dock in Phuket, Thailand, and then head up the Red Sea and through the Mediterranean to the Greek islands.

Friends in California said Scott Adam, 70, had previously discussed the dangers of piracy when navigating the Arabian and Red seas. Adam had considered shipping the boat to avoid the dangers of the trip but decided instead to join a rally of yachts heading to the same location, they said.

The couple, however, apparently decided to break off from the Blue Water Rally, which organized and supported the group of boats headed toward the Mediterranean.

Blue Water Rally organizers released a statement on Saturday that said the Adams chose to take an independent route from Mumbai to Salalah, Oman, and left the rally on Feb. 15. They were hijacked three days later.