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 .
NEW : A second source says two pirates were detained by U.S. forces . Four Americans were killed after pirates hijacked their vessel . Negotiations for their release had been under way , involving the FBI . Two pirates had boarded a U.S. Navy ship Monday for talks .