UPPER DARBY, Pa. (CBS) — A foreign exchange student is accused of building an arsenal ahead of a planned attack at his Delaware County high school.

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Authorities say they uncovered weapons, ammunition, and military-style tactical gear at the home of the suspect’s host family.

An Tso Sun, an 18-year-old student from Taiwan, moved to the U.S. last August to attend Bonner and Prendergast Catholic High School in Drexel Hill.

Police say he told a friend that he was planning to shoot up the school. He’s now been charged with making a terroristic threat.

“One of my teachers saw him getting walked out in handcuffs,” said senior Brian Dunphy.

Dunphy shared math class with Sun during first period, but he said it was Halloween when he noticed his behavior.

“He wore, like, a full military costume. It looked pretty legit,” said Brian. “He did have a fascination with guns. People overheard him talking about guns. I guess the right person told and that’s why they took action.”

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Police say Sun told a friend not to come to school on May 1 because he was going to shoot up the school, and then he said he was kidding.

Investigators later discovered that Sun used his school iPad to look up guns, like an AK-47 and an AR-15.

Police say that he showed classmates a bullet last month, and a video of him with a flamethrower and a mask.

The Archdiocese of Philadelphia said in a statement that they were informed of the alleged threat by a student and called police.

“Archdiocesan schools take the safety and security of every student, teacher, and staff member very seriously. Thankfully, no one was harmed. Incidents such as this demonstrate the importance of all members of the school community working together to maintain a safe environment. School families received information about this matter yesterday afternoon,” the archdiocese said.

Upper Darby arsenal

Police say they recovered an arsenal for a planned school attack from a foreign exchange student’s home. (credit: CBS3)

Upper Darby police showed evidence they say they pulled from Sun’s bedroom, including a military-style ballistic vest, ski mask, military web gear with pouches to hold ammunition clips, a high-powered crossbow and arrows, a bullet loading dock for an AR-15 and AK-47, an empty box for a gun-making kit, 20 live round, ear protectors for firing, and a strangulation device called a garrote.

“He was planning something horrible,” said Upper Darby Police Superintendent Michael Chitwood.

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Sun was arrested from the school and arraigned Tuesday night. He also underwent a mental evalution.

He is currently in Delaware County Jail.

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