WASHINGTON (WASHINGTON POST) - US police have arrested an 18-year-old exchange student from Taiwan for allegedly threatening to massacre his high school classmates.

An Tso Sun, who was arrested on Tuesday night on a terroristic threat charge, assembled the beginnings of a "military-style" arsenal in his suburban Philadelphia bedroom, police said, adding they acted after Sun's behaviour triggered concerns he was planning an attack.

Sun came to the US five months ago, his defence attorney Enrique Latoison told The Washington Post and was enrolled at Bonner & Prendergast Catholic High School in Drexel Hill, Pennyslvania for his senior year.

On Wednesday (March 28), Upper Darby police hauled the ammunition, crossbow, ski mask and gun-making components they found in Sun's room to a news conference.

"When you're able to amass these types of items and you're not even a citizen of the United States … something's wrong somewhere," Police Superintendent Michael Chitwood told the gathered reporters. "Someone's not watching."

While police framed the teen's behaviour ominously, Sun's lawyer said the teen was harmless - a military enthusiast who didn't realise how a dark joke would be interpreted by his classmates, in a country that has been traumatised by school massacres.

"He didn't grow up in the Columbine generation," Latoison said, referring to the 1999 Columbine High School massacre. "He's only been here five months. He doesn't get it like we get it."

Latoison said Sun came to the United States in the fall from Taiwan. He is the only child of Taiwanese TV actors Di Ying and Sun Peng.

Di told Taiwan media that her son's alleged threat to shoot at his school was either a joke or a "misunderstanding" over language.

She said her son has always wanted to build a law enforcement career like that of celebrated detective Henry Lee, and has been accepted by a US university to major in criminal justice, starting in August this year.

"My husband and I are not able to reach our son and I'll go to the US to help him tomorrow," Di was quoted as saying.

Sun was being hosted by the family of a Pennsylvania attorney, whom police have not named, when he was detained.

Latoison said his client had already been accepted to a US university and had planned to major in criminal justice. But according to police, his behaviour in high school alarmed his classmates.

In February, Chitwood said at a news conference on Wednesday, Sun brought a "high-calibre" bullet to school and showed it off to a schoolmate.

He also showed the student a video of himself in a mask, operating a flamethrower, the police superintendent said.

Some time later, Chitwood said, Sun told a classmate: "Don't come to school on May 1, because I'm going to shoot up the school."

Sun told his classmate the remark was a joke, according to both police and the defence attorney. But police don't believe he was kidding.

Later, Chitwood said, Sun asked the same student how to buy a gun. This concerned the student enough that he reported the remark to a teacher, and the high school principal contacted police about it Tuesday morning.

"He's not being bullied; he's not mad at anybody," Latoison told The Post. "He was just thinking he's being funny. And he wasn't."

In any case, police began investigating Sun the same day. They learned he had logged into a school iPad to research AK-47 and AR-15 rifles and had searched for instructions on how to buy the weapons.

Convinced that the teen's threat was serious, police obtained warrants, arrested Sun on Tuesday afternoon and then searched his host family's home.

At the news conference, police had laid out items seized from the teen's bedroom across two long tables.

These included a ballistic vest and military-style ski mask, a backpack with ammo pouches, a crossbow complete with scope and light, and a garrote - like the kind assassins use in movies. "You go up behind somebody and you put it around their neck and you strangle them," Chitwood explained.

While police had not found any firearms in the room, Chitwood held up boxes of ammo, a device for efficiently loading bullets into an AR-15 or AK-47, and what he said was a kit with components for a homemade gun.

"When you add it all together, we believe he was planning something horrible," Chitwood said.

On the contrary, Sun's lawyer said, everything seized from the bedroom was legal and explainable without concocting a massacre plan.

Sun had brought the tactical vest from Taiwan, Latoison said, and had worn it with the backpack and ski mask to school for a Halloween party last year. "He dressed up as a black ops person with that gear. … He's interested in military stuff."

As for the crossbow, the attorney said, "he purchased it here. Any American can get it in any Dick's Sporting Goods."

"It looks bad," Latoison acknowledged. "I know why it looks bad … and unfortunately he won't get a fair shake because of the climate we're in."

But Sun was not a would-be killer, he said - rather an aspiring law enforcement officer with a bad sense of humour.

The teen was imprisoned on US$100,000 (S$131,000) bail on Wednesday. Latoison said his next court hearing will be April 11.