A 28-year-old man on a plane from Japan was arrested in Los Angeles after he was found wearing body armor and flame-proof pants, and transporting a small arsenal of weapons, officials said Tuesday.

Yongda Huang Harris, a naturalized US citizen of Chinese descent, faces a charge of transporting hazardous materials after he was detained by border guards Friday at Los Angeles International (LAX) airport.

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.

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 added.

Agents also 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 (HSI), 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 Inchon, Korea, to LAX."

Bomb squad officers X-rayed the smoke grenade, and found that the "device fell under the United Nations' explosives shipping classification, meaning it is prohibited on board passenger aircraft.

"When ignited with sufficient airspace surrounding it, the smoke grenade would cover an approximately 40,000 cubic foot area with smoke, sufficient to fill the cabin of a commercial airplane," said the affidavit.

"When ignited without sufficient airspace, the smoke grenade was capable of causing a fire," added the document, obtained by AFP.

The affidavit noted that Harris admitted only to having a knife, when questioned about his customs declaration, and before officers went through his luggage.

The CBP agent had "noticed that Harris was wearing body armor (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.

"Harris commented that he had a knife, but did not otherwise seek to amend his declaration," shortly before the contents were uncovered.

Investigators are coordinating with counterparts in Tokyo, it said, adding that Harris has a permanent home in Boston, but "has been living and working recently in Japan."

He was due to appear in court in Los Angeles later Tuesday, and could face up to five years in prison.