Former negotiator believes Niger targeted girls to force concessions . Nigerian security chiefs call on those with information about the girls to come forward . Search must expand to Boko Haram , France and Chad , a U.N. official says . Now , Boko Haram may be going after those trying to find the girls . Britain joins United States , Cameroon , China in search for girls abducted by group .
Former negotiator believes Niger targeted girls to force concessions . Nigerian security chiefs call on those with information about the girls to come forward . Search must expand to Boko Haram , France and Chad , a U.N. official says . Britain joins United States , Cameroon , China in search for girls abducted by group .