A 28-year-old man on a plane from Japan was arrested in Los Angeles after he was found wearing body armour and flame-proof pants, and transporting a small arsenal of weapons, officials said.
Yongda Huang Harris, a naturalised US citizen of Chinese descent, faces a charge of transporting hazardous materials after he was detained by border guards Friday at Los Angeles International Airport (LAX).
Harris, who has lived in Japan recently and who was heading for Boston, was "wearing a bulletproof vest and flame retardant pants underneath his trench coat", said a US Immigration and Customs Enforcement (ICE) statement on Tuesday.
Further investigation of his checked luggage turned up a "pyrotechnic smoke grenade", along with a hatchet, knives, a gas mask, biohazard suits and body bags, it said.
Agents found three leather-coated billy clubs, a collapsible baton, a full-face respirator, leg irons and a device to repel dogs, said an affidavit by officers with Homeland Security Investigations, a division of ICE.
The document accused Harris of "willfully or recklessly transporting a smoke grenade in his checked luggage on a flight from Kansai, Japan, via Incheon, Korea, to LAX".
Bomb squad officers X-rayed the smoke grenade, and found the "device fell under the United Nations' explosives shipping classification, meaning it is prohibited on board passenger aircraft. The affidavit noted Harris admitted only to having a knife, when questioned about his customs declaration, and before officers went through his luggage.
The customs agent had "noticed Harris was wearing body armour (including a bulletproof vest, flame-retardant legging covers, and knee pads), underneath a trench coat".
The agent "spoke with Harris and asked for his customs declaration", and gave him "an opportunity to amend his declaration prior to officers conducting a baggage exam".
Investigators are co-ordinating with counterparts in Tokyo, it said. Harris was due to appear in court in Los Angeles, and could face up to five years in prison.