DETROIT (AP) — A former U.S. Marine held in Iran describes the miserable prison conditions he has endured in a new letter to the U.S. secretary of state.

Amir Hekmati also says Tehran is holding him hostage for use in a possible prisoner exchange, according to the letter addressed to John Kerry and published by Britain's Guardian newspaper on Wednesday.

Family spokesman Greg Romano told The Associated Press on Wednesday that Hekmati's sister, Sarah (SAR'-uh), authenticated the letter. She says the handwriting is her brother's as is the letter's tone.

The AP has sent an email seeking comment from the State Department.

The Guardian says it obtained the letter after it was smuggled out of jail.

Hekmati has been detained on what the U.S. calls false espionage charges. He grew up in Michigan.

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