Edward An-Tso Sun, 18, was arrested in March after another student told school officials that Sun had warned he planned to attack his school in Upper Darby, a suburb of Philadelphia, on May 1.
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"Hey, don't come to school on May 1st.... I'm going to come here armed and shoot up the school. Just kidding," Sun told the fellow student, according to the charges.
When notified by police, Sun's host family recovered from his bedroom the huge cache of ammunition and a semi-automatic pistol. Later police found a full "ballistic suit" including a vest, jacket and pants, and a crossbow.
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"When someone makes an alleged threat of this kind, law enforcement must take it seriously and investigate all leads," said US Attorney William McSwain.
"Law enforcement is committed to doing everything in our power to stop an attack before it happens."
The charges came amid a spate of school shootings that have left dozens dead and injured and made global headlines.
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In the worst cases this year, 10 people were killed when student attacked his high school in Santa Fe, Texas on May 18, and 17 were killed in Parkland, Florida on February 14 after a former student opened fire.
Sun was charged in federal court with one felony count of possessing ammunition while being in the United States on a non-immigrant visa.
The charge brings up to 10 years in prison and a $250,000 fine.
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