A Massachusetts man has been charged with illegally transporting a smoke grenade in his checked suitcase while returning from a trip to Japan.

Agents from Immigration and Customs Enforcement arrested Yongda Huang Harris, 28, a naturalized U.S. citizen of Chinese descent, Friday in Los Angeles. He was flying from Japan, through South Korea and then Los Angeles, on his way to Boston.

What alerted agents to check his suitcase?  It might have been the bullet-proof vest or the flame-retardant pants that he was wearing under his trench coat.

According to ICE, a search of his checked bag turned up the smoke grenade as well as "three leather-coated billy clubs, a collapsible baton, a full-face respirator, various knives, a hatchet, body bags, a biohazard suit, handcuffs, leg irons, and a device to repel dogs."

He appeared late Tuesday in federal court in Los Angeles. There's no thought he was a terrorist or was plotting to do anything on the various planes he was flying on, a federal official says. But transporting a smoke grenade is illegal.

ICE isn't saying what airline allowed him to begin his journey. Court documents say his trip originated in Kansai, Japan, stopped over in Inchon, Korea, and then in Los Angeles on his way to Boston.  A federal official says the Japan to Korea flight was not on a U.S. carrier.

Pete Williams is NBC News' chief justice correspondent. 

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