U.N. officials say they are concerned that Afghan authorities may have tortured suspects to get confessions in the alleged poisoning of hundreds of schoolgirls.

Starting in late May, female students across northern Afghanistan complained about noticing strange smells in their schools before becoming dizzy and nauseous, and even fainting and vomiting in some cases.

No one died in the incidents, and some experts have suggested that a phenomenon called “mass hysteria” against the backdrop of Afghanistan's ongoing conflict could be behind the episodes.

Afghan authorities said they had confessions in the case after announcing the arrests of 15 suspects last month. But the World Health Organization says no traces of poison have ever been found in samples from the students.

A spokesman for the U.N. mission in Afghanistan, Nazif Salarzai, says his office has seen evidence that suggests the suspects confessed after being tortured. He said if that is true, then officials cannot rely on the confessions.

But Afghanistan's home ministry spokesman Sediq Sediqi strongly denies allegations of torture. He also says authorities captured the suspects with what he called “sprays and tablets” — supposedly proof of the alleged poisonings.

Afghan authorities have accused both the Pakistani military spy agency and the Taliban of being behind the incidents. Both groups have denied the allegations.