A foreign exchange student from Taiwan was arrested after allegedly threatening to “shoot up” a Pennsylvania high school, local police say.

An Tso Sun — an 18-year-old in the U.S. on a student visa — is accused of making a threat to another Bonner Prendergast Catholic High School student, saying he was plotting a shooting spree on May 1, according to The Inquirer. Afterward, the foreign exchange student said he was “just kidding.”

Nonetheless, the student whom Sun had made the threat to told authorities and school officials. After the foreign exchange student was arrested by police, his home was raided and authorities found that Sun had been searching how to buy an AK-47 and an AR-15.

“When you add it all together, we believe that he was planning something horrible,” Upper Darby Police Superintendent Michael Chitwood told the local media.

Though police did not find a gun in his home, they did find:

A bulletproof vest

A ski mask

9mm bullets

A crossbow

Earplugs

Sun first entered the country in July 2017. Now, the foreign exchange student is being held at the Delaware County Prison and is charged with making terrorist threats.