Iranian President Ebrahim Raisi on Friday blamed a wave of poisonings of hundreds of schoolgirls around the country on Tehran’s enemies.

The so-far unexplained poison attacks at more than 30 schools in at least four cities started in November in Iran’s Shi’ite Muslim holy city of Qom, prompting some parents to take their children out of school.

Iran’s health minister said on Tuesday that hundreds of girls in different schools have suffered and some politicians have suggested they could have been targeted by religious groups opposed to girls’ education.

Raisi, speaking to a crowd in southern Iran on Friday in a speech carried live on state television, blamed the poisoning on Iran’s enemies.

“This is a security project to cause chaos in the country whereby the enemy seeks to instill fear and insecurity among parents and students,” he said.

He did not say who those enemies were although Iranian leaders habitually accuse the United States and Israel, among others, of acting against it.

Separately, a senior Iranian official said a fuel tanker found next to a school in a Tehran suburb and which had also been spotted in two other cities was probably involved in the poisonings.

Authorities seized the tanker and arrested its driver, Reza Karimi Saleh, deputy governor of Pardis suburb, told the semi-official Tasnim news agency.

He is the first government official to report an arrest in connection with the wave of poisonings.

He said the same tanker had also been to Qom and Boroujerd, in Lorestan Province in western Iran, where students have also suffered from poisoning. He did not elaborate. “Guards at a parking lot where the fuel tanker was parked also suffered from poisoning,” Saleh said, referring to the Pardis site.

In Geneva, the United Nations human rights office on Friday called for a transparent investigation into the attacks.

“We’re very concerned about these allegations that girls are being deliberately targeted under what appear to be mysterious circumstances,” Ravina Shamdasani, spokesperson for the U.N. High Commissioner for Human Rights, told a briefing.

She said the findings of a government investigation should be made public and the perpetrators brought to justice.

Some Iranian politicians have suggested the schoolgirls could have been targeted by religious groups opposed to girls’ education.

Social media posts are replete with photos and videos of hospitalized girls. Some said they were nauseaous and suffered heart palpitations. Others complained of headaches or heart palpitations. Reuters could not verify the posts.

Schoolgirls have also taken part in the anti-government protests triggered by the death in custody of an Iranian-Kurdish woman last September. They have removed their mandatory hijabs in classrooms, torn up pictures of Supreme Leader Ayatollah Ali Khamenei and called for his death.

In one online video last year, schoolgirls are seen waving their headscarves in the air and heckling a member of Iran’s paramilitary Basij force.