Former president believes Boko Haram abducted girls to force concessions . Nigerian security chiefs call on those with information about the girls to come forward . Search must expand to Niger , Mali and Chad , a U.N. official says . Nigeria joins United States , Britain , Germany in searching for girls abducted by terrorists .
Former negotiator believes Boko Haram targeted girls to force concessions . Nigerian security chiefs call on those with information about the girls to come forward . Search must expand to Niger , Cameroon and Chad , a U.N. official says . France joins United States , Britain , China in search for girls abducted by group .