A Japanese diplomat who committed suicide in May 2004 was being blackmailed to provide key intelligence to China, Japanese media reports say.
China allegedly wanted details of flights carrying documents
The unnamed engineer, in charge of communications for Japan's Shanghai consulate, left suicide notes saying he had been threatened by a Chinese man.
The threats concerned his relationship with a karaoke hostess, reports said.
Japan has asked China to clarify the incident, while China described the latest allegations as "groundless".
A spokesman for the Japanese Foreign Ministry, Yoshinori Katori, was quoted by the AFP agency as saying Japan had complained about the matter.
"We believe that, behind the death, there were regrettable acts by Chinese intelligence," he said.
The case could stoke China and Japan's troubled relations, amid rows over natural resources and Japan's attitudes towards its past.
The Weekly Bunshun magazine reported that the man was being pressed to reveal the names of officials in the Shanghai consulate and the flight numbers of airlines that carried confidential documents from Shanghai to Japan.
According to the Yomiuri Shimbun newspaper, the diplomat did not provide any information to China, and wrote in his suicide note "I can't sell out my country".