COEUR d'ALENE -- When Joseph Edward Duncan III told the FBI he killed a Riverside, Calif., boy in 1997, he also admitted his involvement in the death of two Seattle-area sisters a year earlier, a source close to the investigation said. The source, who demanded anonymity, said Duncan admitted he killed Sammiejo White, 11, and Carmen Cubias, 9, when they disappeared after leaving their north Seattle motel room July 6, 1996. Law enforcement officials, who are under a gag order because of potential federal prosecution of Duncan, would not confirm what Duncan allegedly told the FBI, nor would they deny...