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The Nigerian government says it is investigating the authenticity of an audio recording in which a Christian schoolgirl abducted by Islamic extremist rebels Boko Haram appeals for help.

Leah Sharibu, 15, was among more than 100 girls abducted by Boko Haram fighters from the Dapchi Government Science and Technical School in Nigeria’s Yobe State in February. Sharibu is the only one still held, apparently because she refuses to denounce her Christian faith.

In the recording heard Tuesday, a girl identifying herself as Sharibu urges the government to intervene for her release.

Sharibu’s mother, Rebecca Sharibu, told The Associated Press the recording sounds like her daughter’s voice. A photograph of Sharibu wearing a yellow hijab accompanied the recording.

Six of the girls died during the kidnapping. The other 105 were released.