JAPAN - The pesticide methamidophos was detected on the outer packaging of six Chinese frozen dumplings, known as gyoza in Japan, produced on the same day as those that sickened a family in Takasago, Hyogo Prefecture, the Hyogo prefectural police said Sunday.

The police were yet to examine the inside of the packaging and the gyoza contained in the pack, but one of the six bags was found to have two holes in it.

This is the first time that the chemical has been detected on items unconnected to any of the food poisoning cases. The police are investigating how the pesticide came to be on the packs.

According to the police, the six packs in question are all "Chuka de Gochiso Hitokuchi Gyoza" (delicious Chinese style bite-size gyoza), the same as those that caused three members of the Takasago family to suffer food poisoning after eating them.

The tested packs were among 26 that Sojitz Foods Corp., a Minato Ward, Tokyo-based trading firm, voluntarily presented to the police Sunday, following reports of food poisoning.

The six packs of gyoza were returned to the Osaka branch of JT Foods Co., which imported them, on Dec. 28, and were brought in to Sojitz on Jan. 8.

They were kept frozen together in a plastic bag. One of the packs had a hole about 1 millimeter long on its front, and another hole of similar size on the back near a corner. The holes look like they could have been made by a needle, but the inner tray and the gyoza showed no evidence of having been penetrated or scratched by anything, the police said.

According to Japan Tobacco Inc., JT Foods' parent company, a shop in Osaka Prefecture contacted the company on Dec. 27, saying the six packs were sticky and giving off a strong odor. The firm and Sojitz then decided to collect the products and conduct an investigation.

They tested the gyoza in the packs, but did not find anything unusual. The firms also received a report from Tianyang Food Processing based in Hebei Province, China, which produced the gyoza that ruled out any possibility that the packs could have become coated with a sticky substance during the production process.

Meanwhile, a team of five experts dispatched by the Chinese government arrived in Japan on Sunday to investigate the food poisoning cases.

The Chinese experts discussed how to cooperate over the cases with their Japanese counterparts. The Japanese group includes relevant officials from the Cabinet Office, the police, the Foreign Ministry, the Health, Labor and Welfare Ministry and the Agriculture, Forestry and Fisheries Ministry.

The Chinese team is led by Li Chunfeng, vice director of the Import and Export Food Safety Bureau at the General Administration of Quality Supervision, Inspection and Quarantine, China's national quality control bureau.