A foreign exchange student from Taiwan is behind bars in Pennsylvania after being accused of building an arsenal ahead of a planned attack on his Delaware County high school, CBS News reported.

The 18-year-old student, An Tso Sun, threatened to "shoot up" the Monsignor Bonner and Archbishop Prendergast High School on May 1, Upper Darby police said. Sun later said he was joking but was charged with making terroristic threats, they said.

"Sun was found to have a military vest and ammunition at his residence. Kudos to the person who stepped forward and helped to avert a tragedy," Upper Darby Police Department tweeted.

"We have zero tolerance for anyone who threatens to shoot, bomb, or hurt a school. They will be locked up. Period," police tweeted.

Sun moved to the US last August on a five-year student visa 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.

Classmate Brian Dunphy noted Sun's behavior during Halloween. "He wore, like, a full military costume. It looked pretty legit," said Dunphy.

"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," Dunphy said.

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 it was was 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.

Sun was arrested at the school and arraigned Tuesday night. He also underwent a mental evaluation.

He is currently in Delaware County Jail in lieu of $100,000 bail. A preliminary hearing is scheduled for April 11.