An Tso Sun, the foreign exchange student who pleaded guilty in Delaware County Court this week to threatening to "shoot up" a Catholic high school, was back in court Wednesday, this time to face a new federal charge in Philadelphia.

Sun was calm as he was arraigned for possessing more than 1,600 rounds of ammunition while on a nonimmigrant visa. His green prison jumpsuit appeared to swallow the frail teenager, who looked around the courtroom wide-eyed. He spoke quietly with his attorney but did not address the judge.

The federal case is based primarily on the allegations that led to his arrest in Delaware County this spring.

In March, authorities said Sun warned a classmate at Bonner and Prendergast Catholic High School not to come to the Drexel Hill school on May 1 because Sun planned to go on a shooting spree that day. Upper Darby police arrested Sun, 18, who is from Taiwan and was staying with a host family in Lansdowne, and charged him with making terroristic threats.

Sun told police then he was "just joking" about the planned shooting.

“Mr. Sun may have said he was just kidding after he made this threat, but his alleged threat is no laughing matter,” U.S. Attorney William M. McSwain said at a news conference after the arraignment. 

Authorities said their search warrants had uncovered something more sinister.

"When you add it all together, we believe that he was planning something horrible," Upper Darby Police Superintendent Michael Chitwood said at the time of Sun's arrest.

In Sun's bedroom, authorities found a military-style ballistic vest, a crossbow with a scope and light, 20 rounds of 9mm ammunition, a military ski mask, an ammunition clip loader, and a strangulation apparatus called a garrote. On a school-issued iPad, authorities said, they uncovered internet searches about how to obtain parts to make an AK-47 or an AR-15 semiautomatic rifle.

Upper Darby Police Superintendent Michael Chitwood unveils the items recovered in An-Tso Sun's bedroom during a press conference Wednesday morning.
6ABC / Screenshot
Upper Darby Police Superintendent Michael Chitwood unveils the items recovered in An-Tso Sun's bedroom during a press conference Wednesday morning.

Police later discovered a 9mm Glock handgun and 1,600 rounds of ammunition.

Attorneys have said Sun's parents are in the United States to support their son, but the couple did not appear in court or at the news conference on Wednesday. They have been identified by various news outlets, including the Associated Press, as the Taiwanese actor Peng Sun and opera singer Ying Di.

According to the Taiwan News, they have apologized for their son's behavior, insisting any threat must have been unintentional or the result of a language barrier.

"We as parents feel remorseful and guilty," Peng Sun told the News. "We firmly believe that our child's actions were unintentional, but we will accompany our child as he faces the American legal process."

His mother said some of the items found in Sun's room were intended to be part of a Halloween costume, according to the news outlet.

Authorities said Sun's host mother, whom they have not identified, had removed the gun and ammunition from his room after Bonner-Prendergast contacted her about the threat and before authorities searched Sun's room. McSwain skirted questions on Wednesday about whether the host mother could face charges as a consequence. But he did offer a stern warning to parents.

"Wake up and do your job," McSwain said. "Law enforcement is not the first line of defense in these matters. You are."

Enrique A. Latoison, a lawyer who at one time represented Sun's host family, said in March that Sun had the equipment because he was interested in a career in law enforcement. Latoison added that Sun, who had only been in the U.S. for about five months at the time, did not understand the implications of making such a statement, even as a "joke," in a time when school shootings happen with regularity.

Caroline Goldner Cinquanto, Sun's attorney in the federal case, declined to comment Wednesday on whether Sun planned to plead guilty. He did not enter a plea at the arraignment. His attorney in the state case could not be reached for comment Wednesday afternoon.

Sun is being held on a detainer lodged by U.S. Immigration and Customs Enforcement and will be held in federal custody until his next court date. If convicted of the federal charge, Sun would be deported and permanently barred from reentering the U.S.

"Americans should be safe from such terror no matter who or where they are," McSwain said. "One more mass shooting is one too many."