The Daejeon education office earlier said the teacher had requested a six-month leave of absence citing depression on 9 December, but had returned to school just 20 days later after a doctor assessed her as being fit to work.

During her time off work, she had suicidal thoughts, said Mr Yook, citing a testimony that the teacher had provided to police.

Days before the stabbing, the teacher had displayed violent behaviour, including putting another teacher in a headlock, said the education office.

Just like school shooters in America. Sign after sign was ignored.