Dozens of girls from 26 schools in Iran are reportedly being treated for poisoning at hospitals after another wave of apparent toxic gas attacks.
More than 1,000 students have been affected since November. They have suffered respiratory problems, nausea, dizziness and fatigue.
Many Iranians suspect the poisonings are a deliberate attempt to force girls’ schools to close.
But the government has not said whether it believes they are premeditated.
Interior Minister Ahmad Vahidi, who has been tasked by the president with finding the “root cause” of the poisonings, on Wednesday dismissed as “false” a report by Fars news agency that three people had been arrested.
He also accused foreign-based and “mercenary groups” of taking advantage of the situation to wage psychological war and worry people.
Some pupils and parents suggested that schoolgirls may have been targeted for taking part in recent anti-government protests.
At least 26 schools in five cities across Iran were affected by the latest gas poisonings, local media and activists said.