NAGASAKI - Friends of a Japanese teenager who admitted to murdering and dismembering a classmate said she was "very smart" but was sometimes "emotionally unstable", news reports said.
The 15-year-old girl, whose name was withheld as she is a minor, allegedly used a metal instrument to deliver blows to the head of her classmate Aiwa Matsuo, also 15, before strangling her.
The victim's head and left hand had been chopped off, police said.
The incident took place at around 8pm on Saturday in the city of Sasebo, in Nagasaki Prefecture, south-west Japan.
The suspect was arrested after confessing to the crime, police said.
"I did it all by myself," the police quoted her as saying.
The suspect lives alone in Sasebo as her parents reside elsewhere in the city, Kyodo reported the police as saying.
Friends of the suspect described her as "very smart, with emotional ups and downs," Japan Times reported.
"She is a very candid girl, and I used to play tag with her," said a 18-year-old former classmate.
"She showed signs of being emotionally unstable and often started crying when she had an argument with someone," she added.
The victim's body was found on a bed in the suspect's apartment early on Sunday and investigators discovered tools nearby, reports said.
The victim had reportedly gone out to meet friends on Saturday afternoon, but her parents notified the police when she failed to return home at night.
The two girls graduated from the same junior high school.
The incident has sent shock waves through the teaching community in Sasebo due to its similarity with another murder in 2004 in which an elementary schoolgirl killed her classmate, Japan Times reported.
The 2004 killing prompted teachers to focus their efforts on designing school activities that would help students understand the sanctity of life.
In the face of Saturday's incident, many now wonder if those efforts were in vain.
"I feel so sad and frustrated. We have kept calling attention to the value of life, but the message has not been delivered," said the principal of the high school attended by the suspect and the victim.
Violent crime is still relatively rare in Japan but several high-profile cases in recent years have heightened public concern.
In May, the body of a nurse was found in a storage lock-up in Tokyo after it was sent across Japan by parcel delivery in a box marked as a "doll".