U.S. officials now say South Korean authorities thoroughly searched Yongda Huang Harris and his carryon luggage but found nothing suspicious, and he was allowed to board his flight to Los Angeles. Harris was flying home to Boston from Japan with stopovers in South Korea and L.A. But once he touched down at LAX -- he was taken into custody.
ABC News learned that 28-year-old Harris was "puzzled" that he was detained and questioned by investigators for wearing a bulletproof-style vest, telling authorities he had already been stopped in South Korea and Japan and allowed to proceed.
Harris was arrested Friday at LAX after he got off an Asiana Airlines flight. His vest was a Chinese-made knockoff and not an actual ballistics vest. But it's what authorities found in his checked luggage that is most disturbing.
Harris was carrying a smoke grenade strong enough to fill an entire plane with smoke. He was also carrying a gas mask, lead-filled leather-coated billyclubs, knives and a hatchet.
Police say the case is strange and concerning because along with the weapons, agents found a collection of other tools fit for a horror movie: body bags, duct tape, handcuffs, leg irons, a protective suit, even oven mitts and cooking tongs.
Federal investigators now want to know how that suitcase made it on the plane.
"He had to go through screening in those countries to get on the plane," said LAPD Commander Blake Chow. "Why it didn't raise any suspicion with the screeners, we're still trying to figure out."
In fact, police say the only reason Harris was stopped at LAX was because a Customs agent noticed he was wearing the suspicious vest under his trench coat, along with flame-retardant leggings and kneepads.
Harris, who appeared in federal court Tuesday, is not cooperating with investigators. He told them: "Hey, this is a game."
"The red flag for me is what was going on at security in Japan that this fellow was able to get this kind of material onto the plane," said terrorism expert Lenny Marcus.
U.S. officials are working with South Korean authorities to determine how the smoke grenade slipped through screening.
ABC News has also learned investigators searched Harris's laptop computer and found pornographic pictures and writings, and information on the use of terror tactics.
While authorities do not believe Harris is linked to a terrorist organization, they haven't determined a motive. Harris is a U.S. citizen of Chinese descent. Sources say recently he'd been working in Japan, but lives in Boston.
Harris has been charged with one count of transporting hazardous materials, an offense that carries a maximum penalty of five years in prison. He's being held until his next court appearance on Friday.