Broccoli salads provided for school lunch were the cause of massive food poisonings that occurred earlier this month at nine elementary and junior high schools in Iwamizawa, Hokkaido, the prefectural government said Wednesday.

The Hokkaido government said an investigation has found that broccoli salads cooked at the city’s joint school meal cooking center for the lunch of Feb. 9 were contaminated with salmonella and that the same genotype of bacteria was confirmed in stools of those who suffered the food poisoning.

More than 1,500 students exhibited food poisoning symptoms by Tuesday but all are now getting well, according to the city’s education board.