On a cloudless afternoon in this sleepy port city, an 11-year-old girl drenched in blood and clutching a box cutter walked into the lunchroom at her elementary school. Teachers and students froze, assuming the sixth-grader known for her lighthearted nature had gravely hurt herself -- but she quickly dispelled that impression, witnesses said, by uttering a few chilling words: "This is not my blood."

Minutes later, teachers found Satomi Mitarai, a 12-year-old girl, lying in a pool of gore in an empty classroom overlooking the sandy playground at Okubo Elementary School. The 11-year old killer, according to her own admissions as recounted in interviews with school officials and counselors, had led Satomi, remembered for her toothy grin, into the room. The attacker drew the curtains before slitting her victim's throat and brutally kicking the dying girl's head and sides, according to those interviewed.

The killing two months ago marked the latest and one of the most extreme in an extraordinary series of youth crimes in Japan -- including a number perpetrated by children who did not show unusual behavior beforehand. In many of the cases, the children involved seemed to snap without warning, in fits of kireru, sudden acts of rage.

The surge in youth violence has sparked calls for a reassessment of the increasingly violent and sexually charged youth culture in Japan, now exported worldwide through animation, comic strips and video games.

The young killer in Sasebo, whose name is being withheld under Japanese law, was an avid fan of "Battle Royale," a popular teen movie turned Internet game in which students kill one another through blood sport. Although the girl is still undergoing psychological evaluation, she is believed to have been set off by a seemingly minor offense: The victim, one of the girl's closest friends, once called her "overweight" and "prissy" on a Web site.

"What is so scary is that she seemed normal to us in every way," said Masashi Watanabe, head of the Sasebo Children's Counseling Center, whose staff interviewed the girl after the killing. "She did not seem like a troubled girl; there were no warning signs picked up by her teachers or parents. She could have been any of our children."

The youth crime wave is damaging the national sense of personal security in a country so safe that young children often ride subways or walk home through teeming cities unaccompanied by adults.

In recent years -- particularly since 1997, when a 14-year-old boy cut off the head of an 11-year old and left it at the entrance gate of his school -- Japan has experienced a rising tide of serious youth crimes, including arson, assault, rape, manslaughter and premeditated murder.

Incidents of violence on school grounds have increased fivefold in Japan over the past decade to 29,300 in 2002, leading the national Mainichi newspaper to warn of Japanese schoolyards descending "into battlefields." Violence by younger children in particular has risen rapidly, with the number of minors under 14 processed for violent crime increasing 47 percent in 2003 from a year earlier. One study by a children's research institute found that as many as 30 percent of high school and middle school students had experienced sudden acts of rage at least once a month. In response to rising youth crime, Japan lowered the age for criminal prosecution in 2001 from 16 to 14 and might lower it further.

Experts blame the violence on low self-confidence among children, and cite pressures on family life during the country's 13-year economic slump. Finances in Japan, the world's second-largest economy, are on the rise, but years in the doldrums sent divorce, domestic violence and suicide rates soaring, tearing at traditional family life and alienating child from parent.

"In Japan, youth crime is not a problem related to poverty," said Akira Sakuta, a noted criminal psychiatrist. "But rather, you can say it's more related to stress and developmental problems from children feeling they are not wanted or are lacking attention."

Many youths have retreated into the virtual world of the Internet, now easily accessed out of adult view on their cell phones. Children can view popular short animated films -- anime -- such as "Gunslinger Girl," a tale about murderous cyborg schoolgirls in plaid miniskirts.

Japan's top literary prize this year went to "Snakes and Earrings" by Hitomi Kanehara, 20. Shocking youth apathy, sex and violence are central elements of the book, a favorite of young people.

To be sure, violent crime is not the only social ill facing Japanese youths. Suicides by minors in Japan shot up for the fifth consecutive year in 2003, jumping 22.1 percent compared to a 6.9 percent increase for adults over the same period.

An estimated hundreds of thousands of Japanese students, from grade school to college, are suffering from a behavioral disorder known as hikikomori, meaning they are unable to leave their homes or cope with daily life, according to experts and sociologists who have studied the phenomenon.

Thousands of teenagers, mostly girls in large cities throughout Japan, have entered into what authorities describe as voluntary prostitution, marketing themselves to adults through Internet sites accessed by cell phone, mostly to earn money for designer handbags and brand-name clothing.

As society searches for answers, the Japanese tradition of discreet affection is coming under fire. A nationwide public service campaign on subways, trains and television is urging parents to hug their children.

"We are confronting a serious problem of how to reach out to our children and teach them the difference between right and wrong," said Kohichi Tsurusaki, superintendent of the Sasebo Municipal Board of Education.

In a country where parents and children traditionally shy away from expressing their feelings, the power of the virtual world has perhaps had amplified effect, experts said. Children, one government expert said, have become too used to dead characters coming back to life with the touch of a button on a game console. The young killer in Sasebo, for instance, did not appear to grasp fully the fact that she had ended her friend's life, telling the family court that she wanted to apologize to her friend in person for the deed, according to sources familiar with the case.

"Many Japanese children live in small block apartments with no pets and are not exposed to real death," said Takeshi Seto, a specialist in youth crime at Japan's Justice Ministry. "They may not understand the concept as much as they should."

Without doubt, some youth crimes -- such as a 12-year old who sexually mutilated and then pushed a 4-year old to his death off the roof of a parking lot in Nagasaki last year -- involve disturbed children with histories of psychotic behavior.

But many students in Sasebo have commiserated not just with the victim here -- but with her killer. According to school officials, the 11-year-old had been under parental pressure to get better grades and was forced to quit the school basketball team to study harder. Insults from her friend may have seemed slight, but students appeared able to understand the girl's rage.

"I wasn't so surprised," one junior high school girl wrote in an Internet chat for students hosted by NHK TV network. "I have experienced the feeling that I hated someone to an extent that I wanted to kill the person . . . a couple of times."

During another Internet chat organized by a local television station in the nearby city of Nagasaki, a student going by the handle "Arrow of Pain" wrote: "I understand so painfully how the offender felt. I have experienced being lonely, and being disliked . . . and of course forced to do things by my parents."

Sasebo, a city of 240,000 located about 200 miles southwest of Tokyo, was already reeling from the killing in June 2003 of a teenage boy by bullies at a local high school. The community is trying to heal in part by fortifying parent groups, encouraging parent-child conferences, and offering broader counseling to children and teenagers.

Part of the process was a recent memorial for Satomi Mitarai, whose father, Kyoji Mitarai, was the Sasebo bureau chief for the Mainichi newspaper and had lost his wife to cancer. Before his daughter's schoolmates placed large yellow sunflowers on a white altar topped with a large portrait of the slain girl at the local community center, Mitarai, fighting back tears, beseeched students to learn a lesson from his daughter's death.

"Please do not forget that right beside you are people who love you the most," he pleaded. "Please do not forget that there are people who would be very sad if you disappeared, even if not by death. Please treasure your lives."

Special correspondents Akiko Yamamoto and Sachiko Sakamaki contributed to this report.

Schoolmates and relatives attended a service last month for Satomi Mitarai, 12, who was slain by an 11-year-old classmate at their school in early June.