NEW : A second report says two pirates were detained by American forces . Four Americans were killed after pirates boarded their vessel . Negotiations for their release had been under way , involving the FBI . U.S. forces had responded after a rocket-propelled grenade was fired at a U.S. Navy ship about 600 yards away -- and missed -- and the sound of gunfire could be heard on board the Quest , U.S. Navy Vice Admiral Mark Fox has said . Two pirates had boarded a US Navy ship Monday for talks .
NEW : A second report says two pirates were detained by American forces . Four Americans were killed after pirates boarded their vessel . Negotiations for their release had been under way , involving the FBI . Two pirates had boarded a US Navy ship Monday for talks .