Japan is reeling from the latest in a string of horrific juvenile crimes - a 12-year-old schoolgirl stabbed to death by her 11-year-old classmate.
The dead girl was said to be friends with her killer
Satomi Mitarai bled to death at her elementary school on Tuesday after being slashed with a small knife.
Her assailant, who has not been named, tearfully confessed to police.
"I wonder how murder could ever take place between two elementary schoolgirls," Prime Minister Junichiro Koizumi told a parliament committee.
The girl's body was found by a teacher in an empty study room during lunch hour after her killer returned to their classroom covered in blood.
"I am sorry, I am sorry," the pupil at Okubo Elementary School, in Sasebo, Nagasaki prefecture, was quoted by the Yomiuri Shimbun as telling police.
Investigators said the girl told police she and Mitarai had fallen out over messages they sent each other on the internet.
"The incident was harrowing... and beyond our imagination," Mr Koizumi was quoted as saying by the French news agency AFP.
"It is the responsibility of adults to think seriously how we should raise children," he said.
There has been considerable hand-wringing in Japan over youth crime ever since a shocking incident in 1997 in which a 14-year-old boy killed an 11-year-old and placed his severed head outside the gates of his school.
That prompted the country's parliament to lower the age of criminal responsibility from 16 to 14.
Last year a 12-year-old boy in Nagasaki was accused of murdering a four-year-old boy by pushing him off a roof.
The latest incident shocked Japan's media.
"We must make children understand even more the basic importance of life," the Yomiuri said in an editorial on Wednesday.
Although Japan is still one of the safest developed nations in the world, youth crime has dramatically increased in recent years.
The number of children under 14 committing serious crimes in 2003 rose to 212, a 47% increase on the previous year.
Mitarai's killer, who is too young to be punished under the Penal Code, is expected to be sent to a family court which will decide whether to launch a hearing into the case, Kyodo news agency said.