The attackers have confirmed that they are from the Islamist group, Boko Haram
Nigerian Islamist militant group Boko Haram says it carried out the abduction of hundreds of schoolgirls in April.
About 230 girls are still believed to be missing, prompting widespread criticism of the Nigerian government.
"I abducted your girls," Boko Haram leader Abubakar Shekau said in a video obtained by the AFP news agency.
Meanwhile, a woman who helped organise protests over the abduction has been detained, her fellow community leaders say.