MAIDUGURI, Nigeria - At least 12 people were killed on Wednesday when two female suicide bombers blew themselves up in a market in the northern Nigerian town of Chibok where Boko Haram militants abducted more than 200 girls almost two years ago, police and residents said.Boko Haram has been waging a six-year armed campaign in Nigeria's remote north to build an Islamic state. Thousands have been killed and more than two million people displaced by the campaign.There was no immediate claim of responsibility but the attack bore the hallmarks of Boko Haram, which has been using suicide bombers since the army expelled the group from much of the northern territory it had captured previously."Two veiled girls suspected to be suicide bombers entered Chibok market at about 12.54 pm today," a police official said, asking not to be named."Afterwards we heard an explosion at the heart of the market and people fled. Some minutes later another blast came just at the edge of the market," he said.The official said he had counted eight bodies at the market, while four others had died on the way to the hospital. Some 15 people were treated in hospital, he added.Musa Pagu and Ishaya Ali, members of a civilian vigilante group that helps maintain order in the area, also said 12 people had been killed.