(CNN) Former House Speaker Dennis Hastert reported to prison Wednesday to begin a 15-month sentence in a hush money case stemming from allegations that he sexually abused young boys, authorities said.

Hastert, 74, entered the Federal Medical Center in Rochester, Minnesota. He is not facing sexual abuse charges because the statutes of limitation have expired. But he pleaded guilty to hiding money transactions, admitting that he paid $3.5 million to keep one victim quiet

The incidents allegedly happened when he was a teacher and wrestling coach in Illinois in the 1960s and '70s.

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Hastert also acknowledged misleading the FBI last year when the agency was investigating the claims.

Judge Thomas Durkin called Hastert "a serial child molester" when he handed down the sentence in April, CNN affiliate WGN reported.

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