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.