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.