But there were warning signs. City officials said that when she was in her sixth year of primary school, she laced the lunches of two classmates with diluted bleach and detergent on five occasions. A source close to one of the victims said the girl had been angry with remarks about her studies.

The girl's mother apologised at the time, promising to discipline her. She received counselling and the school reported the problem to the municipal government, according to sources. But a member of the city's education board said he believed no official report was made.

A former teacher at the primary school said the city, school and the parents might not have taken the right action.

"We just sought a closure of the problem without seriously facing the dark side of her mind," the teacher said. "I feel sorry because we could've prevented the incident if we had acted in the right way."

The girl's life took a radical turn last autumn when her mother fell ill with cancer and died after being bedridden for two months. Police sources said that after her mother died, the girl tried to kill her father by bashing him with a metal bat, causing serious injuries. But police were not informed at the time.

"Evidently it was a case of attempted murder," an official at the prefectural police said. "If we had known about it, the incident this time wouldn't have happened," he said.

In April, the girl entered senior secondary school and moved out of the family home, living alone in the condominium where the killing took place.

Her father promptly remarried in May.

The girl showed up at school for only three days, including the day of the entrance ceremony. A home-room teacher visited her at her apartment every week but the attempt to reach out to her apparently had no effect.

Clinical psychologist Hirokazu Hasegawa said: "The mother's death and the father's remarriage immediately afterwards are significant events for any child.

"After living separately [from her father], she was in a world of her own. That may have allowed her to further develop her curiosity about dissections."

A police officer said she cut up her victim because she was "interested in trying dissection on a human after dissecting a cat and reading medical books".

Misako Noguchi, an official of Infinity, a Nagasaki-based nonprofit group providing support to parents, said: "There are also other children with similar traits. It's important that people around them take note of any signs children give."