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

An investigation has found that broccoli salads cooked at the city’s joint school meal cooking center for lunches on Feb. 9 were contaminated with salmonella and the same genotype of bacteria was confirmed in the stool of those who suffered the food poisoning.

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