A graduate of the School of Veterinary Medicine was among the four U.S. citizens who were killed Feb. 21 aboard their yacht, four days after Somali pirates allegedly commandeered the vessel off the coast of east Africa and held the Americans as hostages.

U.S. Navy Vice Adm. Mark Fox, in a telephone news conference from Bahrain, said the captors shot the hostages after firing a rocket-propelled grenade at a U.S. ship. Navy personnel soon boarded the yacht, but "despite immediate steps to provide life-saving care, all four of the American hostages died of their wounds," Fox said in his briefing, as transcribed by the Department of Defense and posted online.

The American victims included Robert Riggle, 67, a 1967 graduate of the veterinary school. He was retired, living in Seattle and working as a relief veterinarian for the Seattle Animal Shelter for the last seven to eight years, providing spay and neutering services for adopted animals and through a city program, according to The Seattle Times.

Riggle and Phyllis Macay, also of Seattle, had recently joined Scott and Jean Adam aboard their yacht, the Quest. Riggle and Macay blogged about their adventure at gaiaworldtour.net.

The Adams, of Marina del Rey in southern California, had been sailing the world since December 2004, distributing Bibles along the way.

Vice Adm. Fox, who leads the U.S. Naval Forces Central Command and the 5th Fleet, said four Navy ships had been shadowing the Quest and negotiating with the pirates by radio, and in person with two pirates who came aboard the USS Sterett.

Then, at 8 a.m. local time Feb. 22, the pirates fired on the Sterett, a guided-missile destroyer 600 yards away, Fox said in a telephone news conference from his headquarters in Bahrain. The Department of Defense

The grenade missed, and, almost immediately thereafter, Navy personnel heard small-arms fire coming from the Quest’s cabin, Fox said. Several pirates appeared on deck with their hands up, after which the Navy boarded the vessel.

Navy personnel killed two pirates, according to Fox, who added that the Navy found two other dead pirates on the yacht. The Navy took 15 pirates into custody, including the two who had stayed overnight on the Sterett.

There were no reported injuries to Navy personnel or any damage to the U.S. ships.