Chinese dumplings which were laced with pesticide and made at least 10 Japanese people ill were probably poisoned on purpose, Japan's health minister says.
People said they fell ill and vomited after eating the dumplings
The frozen dumplings were contaminated with a highly toxic organophoshate pesticide methamidophos.
"Judging from the circumstantial evidence, we'd have to think that it's highly likely to be a crime," Yoichi Masuzoe told reporters.
The issue has triggered intensive media coverage and public alarm in Japan.
Japanese police say they want to work with their Chinese counterparts to investigate how the dumplings, known as gyoza in Japan, were poisoned.
The problem emerged last Wednesday, when 10 people were reported to have fallen ill from the dumplings - thin dough packets containing ground meat and vegetables which are then fried.
The dosage of pesticide was sufficiently concentrated to leave one five-year-old girl seriously ill.
By late Thursday there were unconfirmed reports of dozens of other cases.
The Japanese distributor, JT Foods, quickly recalled the dumplings and other products made by the same Chinese company, Tianyang Food.
But China says preliminary tests on two batches of the dumplings found them to be safe.
The two countries have not yet ratified an agreement to allow investigators to travel between them though so any contacts will be made through diplomatic channels.
In the meantime the Japanese will try to rule out the possibility that the dumplings were poisoned in Japan after they had been brought in from China.
In recent months, Chinese-made products have been involved in several safety scares.
But Japan has also had its problems. Last year, confectioners admitted to mislabelling expiry dates on biscuits and rice cakes.