Former president believes Boko Haram abducted girls to force concessions . The assault fits a pattern of revenge-seeking by Boko Haram against those perceived to disagree with the group or those who have provided aid to the Nigerian government . ' 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 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 .