A 15-year-old boy was detained by school staff and later arrested on Monday after a teacher was fatally stabbed in northern England.
Anne Maguire, 61, died after she was taken to the hospital.
"The incident itself was over very quickly, and no other pupils or staff were threatened or injured. However, this has clearly been a very traumatic situation for those involved," chief superintendent Paul Money of West Yorkshire Police said in a statement.
Witnesses told the BBC that some children fled the scene at Corpus Christi Catholic College in Leeds, running and screaming after the attack.
A murder inquiry has now been launched. "Specialist detectives who are highly experienced in treating child witnesses sensitively are being used to interview them," said Money.
Katie Gallagher, 15, told media that she was in a nearby room when Maguire, a Spanish teacher, was hurt.
"I heard her scream. I was in a meeting at the time but I heard it," Gallagher said, according to the London Evening Standard.
Police have said the teenage boy remains in custody.