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PHILADELPHIA (CBS/AP) — A Taiwanese exchange student accused of threatening a shooting at his suburban Philadelphia high school is facing a federal firearms charge.

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Eighteen-year-old An-Tso Sun pleaded guilty Monday in state court to terroristic threats and was sentenced to four to 23 months behind bars. He immediately was paroled to federal authorities, who announced Wednesday he was charged with felony possession of ammunition while in the country on a non-immigrant visa.

“He was planning something horrible,” said Upper Darby Police Superintendent Michael Chitwood after his arrest in March.

School authorities were alerted in March that Sun talked about a May 1 shooting at Monsignor Bonner and Archbishop Prendergast High School, warning a friend not to go to school that day. Sun called it a joke, but prosecutors said ammunition and weapons were found in his home.

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Sun’s attorney hasn’t returned a call seeking comment.

(© Copyright 2018 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)