In response to the public’s outrage after a slew of alleged school attacks, Iran’s supreme leader was quoted as saying on Monday that poisoning schoolgirls is a “unforgivable” act that, if intentional, should result in death.

State media and officials report that more than 1,000 girls have become ill after being poisoned since November, and some politicians have blamed religious organizations that oppose girls’ education.

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.

“Authorities should seriously pursue the issue of students’ poisoning,” Ayatollah Ali Khamenei was quoted as saying by state TV. “If it is proven deliberate, those perpetrators of this unforgivable crime should be sentenced to capital punishment.”

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

Authorities have accused the Islamic Republic’s “enemies” of using the attacks to undermine the clerical establishment. But suspicions have fallen on hardline groups operating as self-declared guardians of their interpretation of Islam.

The poisonings were disgraceful, according to the press secretary for President Joe Biden in Washington on Monday.

Karine Jean-Pierre remarked during a news briefing that the notion that Iranian girls may be poisoned just for attempting to pursue an education is disgusting and wrong.

If the poisonings were tied to protests, the White House demanded a separate probe, which would fall under the purview of the United Nations fact-finding mission on Iran.