Iran’s top leader said Monday that poisoning schoolgirls is an “unforgivable” crime that should be punishable by death if it is intentional, state TV reported, amid public anger over a wave of suspected attacks in schools.

More than 1,000 girls have been poisoned since November, according to state media and officials, with some politicians blaming religious groups opposed to girls’ education.

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The poisonings have come at a critical time for Iran’s clerical rulers after months of protests since the death of a young woman held by police for flouting hijab rules.

“The authorities should seriously pursue the issue of the poisoning of the students,” Ayatollah Ali Khamenei said, according to state television. “If proven intentional, those who committed this unforgivable crime should be sentenced to death.”

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The UN Human Rights Office calls for a transparent investigation into Iranian schoolgirls

The poisonings began in November in the holy Shiite city of Qom and spread to 25 of Iran’s 31 provinces, prompting some parents to pull children out of school and protest.

Authorities have accused the Islamic Republic’s “enemies” of using the attacks to undermine the clerical establishment. But suspicion has fallen on hard-line groups that act as self-proclaimed guardians of their interpretation of Islam.

In 2014, people took to the streets in the city of Isfahan after a wave of acid attacks, which appeared to be aimed at terrorizing women who violated the strict Islamic dress code.

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For the first time since the 1979 Islamic revolution, schoolgirls have joined the protests that spiraled after Mahsa Amini’s death in moral police custody.

Some activists have accused the establishment of the poisonings as revenge.

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“Now the girls of Iran are paying the price for fighting the mandatory hijab (veil) and have been poisoned by the clerical establishment,” tweeted New York-based leading Iranian activist Masih Alinejad.

Fearing new impulses for protests, the authorities have downplayed the poisonings. A legal investigation is ongoing, but no details of the findings have yet been released.

At least one boys’ school has also been targeted in the town of Boroujerd, state media reported.