Several people in Iran have been arrested over a wave of suspected poison attacks at girls schools that have hospitalized thousands of students since they were first reported four months ago, a senior government official said.

Deputy Interior Minister Majid Mirahmadi didn’t give a precise number for those detained and told Iranian state TV on Tuesday that the arrests were made in five different provinces, without being specific.

He said the incidents — which have sparked fresh protests at a time when Iran’s Islamic leadership faces unprecedented levels of popular dissent — have dropped significantly since Monday.

Mirahmadi’s statement comes a day after Iran’s Supreme Leader, Ayatollah Ali Khamenei, addressed the attacks for the first time and called them a “big crime” that should be “severely punished.”