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Tom Blank, a former deputy administrator at the TSA, said the U.S. likely will look at whether the failure to detect the grenade on a U.S.-bound jet was a one-time lapse or part of a wider security vulnerability.

If the U.S. determines a country’s airport doesn’t meet U.S. standards, it can ask for stronger security measures and even prohibit flights from flying directly to the U.S. from that country.

“This clearly looks like an error. Something slipped through that should not have slipped through,” Mr. Blank said of the grenade.

There is no indication that Mr. Harris, who does not have a criminal record, is linked to a terrorist organization or planned to damage the plane, and it’s not likely a smoke grenade could bring down the aircraft, the federal official said.

But the smoke grenade is banned from planes under the United Nations’ explosives shipping rules. Depending on the conditions when it is ignited, the grenade could fill the cabin with smoke or cause a fire, officials said.

Asked about the grenade, the Korean airport security official pointed out that South Korean guidelines list as legal nonflammable, inert cartridges or tins that produce smoke, but said he would have to see the specific item before he could say more.

Customs officers also believed that the lead-filled, leather-coated billy clubs and collapsible baton might be prohibited by California law, according to an affidavit filed in U.S. District Court.

Mr. 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.

Mr. 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 Mr. Harris‘ family in Boston were unsuccessful. His attorney, Steven Seiden, was unavailable, said spokesman Chris Williams, who described Mr. Harris as very intelligent, earning A’s in high school and college calculus.

Mr. Williams objected to characterizations that Mr. Harris was not cooperating with authorities, saying he followed his attorney’s advice to exercise his constitutional right to remain silent.

“He has that right. He is an American citizen,” Mr. Williams said Wednesday.

Mr. Harris graduated from Boston University’s Metropolitan College in January 2011 with a bachelor of science in biomedical laboratory and clinical sciences, said Constance Phillips, the program director. She called Mr. Harris a shy, good student who completed several internships performing research at well-known labs in the Boston area.

Mr. Harris traveled from Kansai, in western Japan, to Incheon before landing in Los Angeles. South Korean officials said he traveled on all Asiana Airlines flights, though the airline declined to confirm that, citing South Korean law banning disclosure of passengers’ information without their consent.

Security at Japanese airports is similar to security in the U.S. Metal detectors and X-rays screen every person and every bag, both checked and carry-on. Airport and immigration officials at Kansai International Airport said Wednesday that airlines are primarily responsible for luggage inspection, but no problematic cases have been reported recently.

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