Japan Today reports that 840 elementary and junior high school students have missed class in Iwamizawa, Hokkaido, due to food poisoning after eating school lunch last week.

Health authorities suspect salmonella infection but have yet to determine what made them sick, they said.

The students started exhibiting the symptoms on Friday after eating lunch the previous day which was cooked in the school kitchens for some 3,100 students.

The lunch included potato miso soup, komatsuna leaf salad and Japanese radish with ground meat, the officials said.