A pirate fired a rocket-propelled grenade at a U.S. Navy destroyer shadowing a hijacked yacht with four Americans aboard Tuesday. Then gunfire erupted, the military said. U.S. special forces rushed to the yacht only to find the four Americans fatally wounded.

The experienced yacht enthusiasts from California and Washington are the first Americans killed by Somali pirates since the start of attacks off East Africa several years ago. One of the American couples on board had been sailing around the world since 2004 handing out Bibles. Their deaths appear to underscore an increasingly brutal and aggressive shift pirates have been showing toward hostages. A pirate who said his name was Muse Abdi said killing hostages "has now become part of our rules," and he referred to a pirate who was sentenced in a New York court last week to 33 years in prison for a 2009 attack on the U.S. cargo vessel the Maersk Alabama. "From now on, anyone who tries to rescue the hostages in our hands will only collect dead bodies," he said. "It will never ever happen that hostages are rescued and we are hauled to prison." Pirates had hijacked the 58-foot yacht Quest south of Oman on Friday.

Since then, four U.S. warships and sky-high drones shadowed the vessel's movement as pirates tried to sail it to the Somali shore. U.S. officials negotiated with the captors via radio. But at 8 a.m. East Africa time Tuesday, a rocket-propelled grenade was fired from the Quest at the USS Sterett, a guided-missile destroyer 600 yards (meters) away. The RPG missed and almost immediately afterward small arms fire was heard coming from the yacht, said Vice Adm. Mark Fox, commander of the U.S. 5th Fleet in Bahrain. Several pirates then appeared on deck with their hands up. U.S. naval forces boarded the vessel and tried to provide lifesaving care to the Americans, but they died, Fox said. No U.S. forces were injured or killed.