One of the four U.S. citizens taken hostage and fatally shot by Somalian pirates this morning was a retired veterinarian. The incident occurred while U.S. military attempted to negotiate a rescue.

Dr. Robert Riggle, of Seattle, Wash., was a 1967 graduate of the University of California-Davis School of Veterinary Medicine. Officials with the program are attempting to find more information about him to notify his classmates.

Riggle, 67, was aboard a yacht that was hijacked by pirates on Friday off the coast of Oman. According to U.S. military officials, Riggle and three others were on an around-the-world sailing trip. Owners of the boat, Jean and Scott Adam, as well as Phyllis Macay, also of Seattle, were killed as pirates sailed the vessel toward Yemen and northern Somalia. 

Riggle and Macay blogged about their adventures sailing the world at www.gaiaworldtour.net.

The incident ranks as the deadliest involving U.S. citizens and Somalian pirates, despite the fact that pirates have terrorized the waters off East Africa for nearly a decade. Four Navy warships had been shadowing the yacht for three days and conducting negotiations for the release of the captives. Four pirates are dead and 15 are in custody, Navy officials report.

Vice Adm. Mark Fox, commander of the U.S. Navy 5th fleet, held a news conference by phone, expressing concern for the "growing problem here in terms of pirate activity off the coast of Somalia."

"The loss of our fellow Americans is a tragedy," he stated.