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MOSCOW (Reuters) - Russian investigators opened a criminal case into a knife attack at a school in the city of Perm on Monday which left 15 people injured, three of whom the authorities said were in a serious condition in hospital.

Media reports initially said two masked men armed with knives had burst into the school and attacked pupils, but authorities in Perm, about 700 miles north-east of Moscow, said the attack grew out of a knife fight between two pupils.

Local security authorities said in a statement that other pupils and a female teacher had tried to break up the fight.

They said twelve people had been taken to a local children’s hospital and a further three, including the teacher and two pupils aged 15-16, were in a serious condition at a different hospital.

Investigators said they had detained the suspects. The school, where almost 700 children studied, had been evacuated, the authorities said.