UPPER DARBY, Pa. (CBS) — Upper Darby police say a foreign exchange student who was arrested last week for threatening to shoot up his school had built a gun with over 1,600 rounds of ammunition.

Police say 18-year-old An Tso Sun of Taiwan, a foreign exchange student at Monsignor Bonner Archbishop Prendergast Catholic High School, had hundreds of rounds for an AR-15 and AK-47 and a 9 mm handgun he built himself from parts purchased online. The parts are virtually untraceable. He was allegedly planning to unleash carnage on the school May 1.

Court records show Sun told a classmate about the possible attack, who in turn blew the lid off the plan once he alerted a school employee.

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According to Upper Darby Police Superintendent Michael Chitwood, Sun had approximately 225 rounds of 12-gauge ammo, 663 rounds of 9 mm ammunition, 295 rounds of AR-15 ammunition, and 425 rounds of AK-47 ammo, along with other sorts of ammunition.

Another troubling revelation released during Monday’s news conference was that once detectives informed Sun’s host parent that he was under investigation, the bag which had been in a bedroom at an East Lansdowne residence was moved in an attempt to hide it.

Police declined to identify the host parent and there is no word if charges are pending for that person.

The woman, whose identity we’re withholding, wasn’t home when CBS3 came knocking. A woman at her law firm took a message for us.

“In my opinion, based on the totality of everything, we certainly feel he was going to shoot down the school,” said Chitwood.

Chitwood says they are still conducting their investigation and if there were other people involved.

The Archdiocese confirms to CBS3 Sun has since been expelled.

Counselors will be available to students on Tuesday.

Robert C. Keller and Michael N. Kotik are representing An-Tso Sun in this Delaware County criminal case.

They released this statement to CBS3:

“Our client just turned 18 in January and is a senior in high school. This young man has had no prior contacts with the courts and has been considered an affable, precocious student with no discipline or antisocial tendencies. The young man’s parents have flown in from Taiwan and fully support their child. Our investigation reveals that there was no intention to terrorize or otherwise commit any criminal act. The family is concerned by the rising level of rhetoric that is being transmitted to the news media and hopes that the public will keep an open mind as all the facts and circumstances are brought forward.”

JUST IN – Statement from attorneys representing An-Tso Sun, ex-Bonner Prendie student charged with threatening to shoot up the high school. FULL report at 5:30 on #CBS3 @CBSPhilly pic.twitter.com/9CbO5qKADU — Joe Holden (@JoeHoldenCBS3) April 2, 2018

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Sun has been charged with making terroristic threats and remains in custody.

U.S. Immigration and Customs Enforcement has a detainer on Sun.

Homeland security will be meeting with Upper Darby Police about the case Tuesday.