(Adds police task force, finding of different insecticide)
TOKYO, Feb 5 (Reuters) - Japan's health minister raised the possibility on Tuesday that someone had deliberately contaminated Chinese-made dumplings imported into Japan with pesticide in an incident that made 10 Japanese sick and sparked a food scare.
Japanese police have set up a joint task force to investigate the case on suspicion of attempted murder.
Both Tokyo and Beijing have called for close cooperation in investigating the case, which has prompted huge Japanese media coverage and health queries from nearly 4,000 people.
The mystery of the poison dumplings is a delicate matter for sensitive Sino-Japanese ties and a domestic headache for Prime Minister Yasuo Fukuda, given criticism from media and opposition lawmakers that it took too long to alert the public.
"The circumstantial evidence suggests substantial possibility of criminality," Masuzoe told a news conference.
Investigators have yet to identify the source of the contamination in the latest furore over the safety of Chinese products ranging from tainted pet food to toothpaste.
Chinese Foreign Ministry spokesman Liu Jianchao called on Japan and the Japanese media to look at the issue "calmly, scientifically and responsibly", and not jump to conclusions.
"Only a calm analysis of the whole situation will be beneficial to the investigation," he told a news conference.
Chinese food quality officials have said that sample tests on the frozen dumplings, ingredients and packaging at the factory concerned had found no pesticide. Liu added further probes had also not turned up pesticide in samples taken.
Japanese experts have said the chemical first detected in the dumplings is used widely in China but not in Japan, although Kyodo news agency said China banned its use this year.
However, Japanese Chief Cabinet Secretary Nobutaka Machimura told reporters it was too soon to tell if the contamination had been deliberate.
"I don't think we should hastily say that this is a (criminal) incident or the like at a stage when the facts are not yet clear," he said.
Japanese police declined to rule out the possibility of criminal intent, but Kyodo said police in two prefectures where the 10 victims -- including a five-year-old who temporarily fell in critical condition -- would jointly investigate the case on suspicion of attempted murder.
Japanese officials are in China to look into the case, which became public when a food trading subsidiary of Japan Tobacco Inc last week recalled products made at the factory that produced the frozen dumplings.
Other firms followed suit, and restaurants, schools and even the Japanese military pulled products from their menus.
Chinese officials have travelled to Japan to help investigations, as both countries seem eager to keep from harming two-way ties ahead of a high-profile visit by Chinese President Hu Jintao this spring, the first such trip in a decade.
As of Tuesday, 3,732 people had made inquiries to health officials about possible illness from eating the products, but no fresh cases of illness had been confirmed, Machimura said.
Kyodo quoted Tsuyoshi Yoneda, head of Japan's National Police Agency's criminal investigation bureau, said his agency wanted Chinese police to cooperate in probing the affair. No Japanese police are included in the team currently in China. (Additional reporting by George Nishiyama and Teruaki Ueno in Tokyo and Ben Blanchard in Beijing; Editing by Bill Tarrant)
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