Japanese hysteria about a rising wave of youth crime was confirmed when a 12-year old girl was stabbed to death in her primary school by an 11-year old classmate.

Satomi Mitarai bled to death on the floor of a classroom in Nagasaki after being stabbed in the neck with a box-cutting knife.

The attack was discovered when her 11-year old assailant returned to her afternoon class still covered in the dead girl's blood.

Ms Mitarai was dead by the time an ambulance arrived at Okubo Elementary School in the town of Sasebo, in Nagasaki prefecture. The killer, who has not been named, was quickly taken into custody and reportedly admitted to the crime.

"This incident is beyond imagination," Hiroyuki Hosoda, Japan's chief cabinet secretary, told reporters. "The details are being investigated."

The murder will reopen an anguished national debate about violent crimes committed by young people, and the legal sanctions which can used against them.

Under Japanese law, criminal responsibility begins at the age of 14 - the girl who murdered Satomi Mitarai was being questioned on a voluntary basis last night, and she cannot be prosecuted in an adult court.

Japan's faith in its young people has been shaken by a string of appalling incidents which have convinced many people that the country is in the grip of a juvenile crime epidemic.

It began in 1997 with what is still one of the most dreadful crimes of all - an 11-year old boy, whose decapitated head was placed in front of the gates of his school.

The murderer, who killed another schoolgirl, left taunting notes for police who assumed that they were dealing with a sophisticated adult killer.

He turned out to be just 14, and was released from a child reformatory earlier this year. Horror and anger at the case spurred Japan's parliament to lower the age of criminal responsibility from 16 to 14.

Since then, it seems, every few weeks have brought a new outrage, luridly recorded by the media. Last July, just 30 miles from the site of today's murder, in the city of Nagasaki, a 12-year-old boy threw a four-year toddler from a multi-storey car park.

Remarks by a junior government minister named Yoshitada Konoike give an idea of how high feelings were running.

Speaking of the parents of the Nagasaki killer, who apparently had no idea of their son's sadistic proclivities, he said that they "should be dragged around town and beheaded."

But crime statistics show that Japan is still one of the safest advanced societies in the world, and that rates of serious youth crime are among the lowest in the world. Burglary and robbery, including armed robbery, are increasing. But murder, by any global standard, is exceptionally unusual.

"Homicide rates in Japan have been going down since the end of the Second World War," says Mariko Hasegawa, a behavioural scientist at Tokyo's Waseda University.

"The mass media focuses on these terrible crimes committed by a few very weird and mentally damaged individuals. But they make out that these are normal people, and that anyone can do such a thing. In fact it is very, very rare."