An 11-year-old girl's admission Thursday that a spat over the Internet provoked her to kill a classmate and a TV program prompted her to use a box-cutter as a weapon had Japanese questioning whether adults were being vigilant enough.

Defense lawyers said the sixth grade suspect in juvenile detention told them that she led 12-year-old Satomi Mitarai to an empty classroom during their lunch hour Tuesday, slit her neck and arms with a box-cutter, and left her to bleed to death.

Lawyer Nozumu Kawazoe told reporters the 11-year-old came up with the idea days earlier while watching a murder-mystery show.

All signs pointed to a close friendship between the girls, who were in art class together, played basketball together, shared a group diary and passed notes on a home page bulletin board.

But Mitarai angered the girl when she wrote negative Internet messages about her appearance, the Kyodo News Agency (search) reported. The girl told police she decided to kill Mitarai when a final message commented on her weight. That message came four days before the killing, the report said, citing defense lawyers and investigative sources.

The sudden turn in the girl's behavior -- who has been described as a cheerful child who caused little previous trouble -- sparked concerns that the problem may be broader.

"Over a computer ... you can't see the person's face, so it's easier to use increasingly violent language. If that's the case, it's an incident that reflects a pathology of society in the age of the Internet," the Tokyo Shimbun, a major metropolitan newspaper, said in an editorial Thursday.

Other national papers ran stories on the surging use of the Internet by children.

In Japan, 62 percent of children between the ages of six and 12 have access to the Internet, according to the Telecommunications Ministry.

The Education Ministry has also pushed in recent years to get more computers and Internet-usage into classrooms, while some schools have begun teaching children "Netiquette," such as respecting other users and encouraging polite conduct during Web interactions.

The gruesome slaying raised doubts about that emphasis on technology.

"What children need most is to be able to piece together real things and real experiences," wrote Hisashi Sonoda, an Internet crime expert at Konan University (search), in an analysis published by the Sankei Shimbun, a conservative daily.

Distressed officials at Okubo Elementary School, in Sasebo, 650 miles southwest of Tokyo, said they examining how the killing could have happened and reviewing student supervision guidelines.

This week's incident has reinforced a rising trend in juvenile crimes and violence at schools in recent years that has eroded Japan's image as a safe haven.

Three years ago, lawmakers lowered the age of criminal responsibility to 14 from 16 amid public outrage over the beheading of a 10-year-old boy by a 14-year-old in 1997.

Police statistics also show that while violent crimes, like murder and robbery, among children under age 14 is still low, it is on the rise.

The suspect's case is being handled by a family court judge who will decide whether to put her on trial. The girl -- whose name hasn't been released, according to Japanese juvenile criminal law -- will not be given psychological testing, lawyers said.

Kawazoe said the girl was visibly troubled when asked about why she killed her friend.

"She said she didn't know why she did it. ... She said she wouldn't have done it if she had given it more thought," Kawazoe said.

"The girl may have given some signals, but the greater tragedy is that adults may have overlooked them," said the Tokyo Shimbun editorial. "Ultimately, it's our responsibility as adults to protect children from incidents and crimes."