A Japanese man who pretended to have a bomb on a U.S.-bound jetliner told police Thursday he repeatedly added "this is a joke" before the pilot turned back to Tokyo as a precaution, officials said.

Police investigators are studying whether they can charge the man, identified as Hiromu Kato, 48, said an official of the Chiba Prefectural (State) Police, speaking on condition of anonymity.United Airlines Flight 58, with 331 people aboard, landed at Tokyo's international airport at Narita shortly after midnight, five hours after taking off for San Francisco and about three hours after turning back. No one was hurt, officials said.