NAIROBI, Kenya (AP) -- Four Americans taken hostage by Somali pirates off East Africa were shot and killed by their captors Monday, the U.S. military said, marking the first time U.S. citizens have been killed in a wave of pirate attacks plaguing the Gulf of Aden and the Indian Ocean for years.

U.S. naval forces quickly boarded the captured yacht after hearing the gunfire and tried to provide lifesaving care to the Americans but they died of their wounds, the U.S. Central Command said in a statement.

Two pirates died during the confrontation and 13 were captured and detained, the U.S. Central Command said from Tampa, Fla. The remains of two other pirates who were already dead for some time were also found. The U.S. military didn't state how those two might have died.

Negotiations had been under way to try to win the release of the two couples on the pirated vessel Quest when the gunfire was heard, the U.S. military said.

The Quest was the home of Jean and Scott Adam, a couple from California who had been sailing around the world since December 2004. The two other Americans on board were Phyllis Macay and Bob Riggle, of Seattle, Washington.