Kano , Nigeria ( CNN ) Troops from Niger and Chad discovered a mass grave with more than 90 decomposed bodies near a northern Nigerian town recently retaken from the Islamic militant group Boko Haram , security sources from both countries said Friday . The corpses , some of them beheaded , were found in a shallow mass grave under a bridge just outside the Nigerian town of Damasak , near the border with Niger , the sources said . Troops from Niger and Chad took back the town from Boko Haram last week during a joint operation , the security sources said . Damasak is in Borno , one of several northern states inhabited by the Islamist militant group . Borno is in the northeast corner of Nigeria and borders Niger , Chad and Cameroon . Boko Haram has terrorized northern Nigeria regularly since 2009 , attacking police , schools , churches , mosques and civilians . It has kidnapped students , including more than 200 schoolgirls who were abducted in April 2014 and remain missing . The group , its naming meaning `` Western education is forbidden , '' has said its aim is to impose a stricter form of Sharia law across Nigeria . Journalist Aminu Abubakar reported from Kano , Nigeria . CNN 's Melissa Gray wrote and reported from Atlanta .
The shallow mass grave is under a bridge near the town of Damasak . More than 90 decomposed bodies . Troops retook the town from Boko Haram .