Reports of unexplained poisoning incidents at girls' schools across Iran have been skyrocketing in recent days.
Since November, girls have reported falling ill with respiratory problems, nausea, lethargy and headaches in Iranian schools, often after smelling a strange odour.
Many have ended up in hospital, and Iranian officials have stated that a "combination of gases" is likely to have been used in some cases.
But the true scale of the phenomenon is difficult to ascertain. Iran allows few international journalists into the country, which makes incidents hard to verify.
Iranian media have stated that 127 schools have been affected, while a prominent politician has said girls at as many as 230 schools have fallen ill as a result of the suspected attacks.
Activist groups have put that figure even higher. Human Rights in Iran has stated that 290 schools have been affected, while activist network 1500tasvir said it has received unconfirmed reports of over 300 separate incidents over the weekend alone.
Using information collected by the Critical Threats Project of the American Enterprise Institute, Sky News has mapped reported poisoning incidents across Iran.
The first reported incident was at a girl's high school in Qom province, Iran's religious centre.
According to local media, 18 students were treated in hospital for what reports said was the release of a "poisonous gas". Just two weeks later, a further 51 students at the same school reported similar symptoms.
In an interview with the regime-affiliated ISNA news agency, one student described losing feeling in their legs after the incident while others spoke of having difficulty breathing.
Sporadic cases have been reported since the incidents in Qom. But it's only in recent weeks that the number of reported attacks has increased dramatically.
According to the Critical Threats Project, which has mapped the number of incidents shared with the activist group 1500tasvir, 46 alleged poisoning cases were reported across four provinces on 1 March.
Just three days later, reports from 16 different provinces across Iran suggested students had experienced poisoning attacks.
As the number of reported cases climbs, footage showing large groups gathered outside schools alongside ambulances as well as young women being treated in hospital has flooded the internet.
This video, reportedly taken on 4 March in Western Azerbaijan province, shows a typical scene.
Sky News has not been able to independently confirm the veracity of the video, although separate reporting indicates an alleged poisoning did occur at this school.
On 5 March, 22 provinces were reported to have experienced suspected poisoning attacks.
The provinces reported to have been affected by poisoning incidents continued to climb on Monday 6 March.
Incidents were reported in 23 out of Iran's 31 provinces, according to the Critical Threats project.
It also said that 6 March saw the first reports of suspected attacks against boys. Until then, cases had predominantly affected girls schools.
What's could be causing these attacks?
There is very little evidence to suggest what could be causing these incidents.
Several analysts approached by Sky News about this issue declined to comment, explaining that the absence of reliable facts make it very difficult to speculate.
The spate of reported incidents, though, comes six months after the wave of nationwide protests following the death of 23-year-old Mahsa Amini in police custody.
Iranian officials have said they are investigating, with Ayatollah Khamenei describing the cases as "unforgivable" and declaring that those responsible will receive the death penalty if found to be carrying out attacks deliberately.
But experts at the American Enterprise Institute's Critical Threats Project believe it possible that the regime could be tolerating a country-wide poisoning campaign.
"It is difficult to imagine that the regime does not have video footage of the individuals responsible for these poisonings," said Annika Ganzeveld, Iran analyst at the Critical Threats Project.
"Many Iranian students have been told that CCTV cameras near their schools were not functioning at the time of the poisonings, which indicates that the regime may be attempting to protect the responsible individuals," she told Sky News.
"The regime has also taken very few steps to protect Iranian students in the aftermath of reported poisonings," she said, although she also pointed out that Iranian law enforcement has said it will increase its presence around schools.
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