That's what federal investigators are trying to figure out despite a lack of cooperation from 28-year-old Yongda Huang Harris, who was arrested during a stopover at Los Angeles International Airport, U.S. Immigration and Customs Enforcement officials said Tuesday.
Harris, who was taken into custody at the airport Friday wearing the vest and flame-resistant pants, was not cooperating with federal officials attempting to interview him, according to a U.S. official who spoke on condition of anonymity to discuss an open investigation.
The official said Harris is not believed to be linked to a terrorist organization, but his motive has not been determined.
Harris has been charged with one count of transporting hazardous materials, an offense that carries a maximum penalty of five years in prison. He made a brief court appearance Tuesday, but his arraignment was delayed until Friday and he was ordered held until then.
Harris is a U.S. citizen whose permanent residence is in Boston, though he recently started living and working in Japan, officials said. Attempts to reach Harris' family in Boston and his associates were unsuccessful.
His attorney, Steven Seiden, was unavailable to comment, said Chris Williams, a spokesman for Seiden, who also represents Mark Basseley Youssef, the man behind the anti-Islam video that recently sparked violence in the Middle East.
It's unclear what Harris had on his body and what he had checked in baggage, which will be crucial information to the defense, said Williams, who declined to comment on why Harris was carrying any of the weapons.
"It raises a lot of questions, and those questions will need to be answered. Right now, the case is very early," Williams said.
The defense attorney's spokesman described Harris as "very intelligent," earning A's in high school and college calculus.
Harris drew suspicion when U.S. Customs and Border Protection officers at the airport noticed he was wearing the protective vest and pants under his trench coat, triggering a formal investigation by Homeland Security special agents.
A search of Harris' checked luggage uncovered numerous suspicious items, including knives, body bags, a hatchet, a collapsible baton, a biohazard suit, a full-face respirator, billy clubs, handcuffs, leg irons and a device to repel dogs, authorities said.
The smoke grenade was X-rayed by police bomb squad officers, who said the device fell into a category that is prohibited on board passenger aircraft.
Such a grenade "could potentially fill the cabin of a commercial airplane with smoke or cause a fire," federal officials said in a news release.
Many of the other items in Harris' luggage—including the hatchet and knives—wouldn't violate Transportation Security Administration guidelines for what is permissible in luggage that is checked.
However, customs officers Kenny Frick and Brandon Parker believed in their initial investigation that the lead-filled, leather-coated billy clubs and a collapsible baton may be prohibited by California law, according to an affidavit filed in U.S. District Court.
A customs official said Tuesday night that Harris was not enrolled in any of the U.S. government's trusted traveler programs, which could have allowed faster processing through security or customs. The official spoke on the condition of anonymity because he was not authorized to discuss details of the ongoing investigation.
Harris traveled from Kansai, in western Japan, to Incheon, Korea, before landing in Los Angeles.
An immigration officer at Kansai International Airport, Masahiro Nakamoto, said authorities reported nothing suspicious at the time the man was believed to have boarded. Spokesman Keisuke Hamatani said Kansai security officials had not reported spotting any suitcases containing the hazardous materials allegedly found in Harris' luggage.
Nakamoto said arriving passengers are checked more closely than those leaving the country.
Yasunori Oshima, an official at Japan's Land and Transport Ministry's aviation safety department, said there had been no official inquiry or request from U.S. authorities to look into the case, which he said would have been more of a concern if the hazardous materials were brought on board rather than checked.
"The case does not seem to pose any immediate concerns about aviation security measures in Japan," he said.
Airport police said they do not believe the case constitutes illegal conduct under the Japanese domestic criminal code, but Japan may cooperate at the request of U.S. investigators.
Spagat reported from San Diego. Associated Press writers Rodrique Ngowi in Boston, Eric Talmadge and Mari Yamaguchi in Tokyo and Alicia A. Caldwell in Washington contributed to this report.