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Tokyo - A 15-year-old Japanese schoolgirl has confessed to decapitating a classmate and cutting off her hand, police said on Tuesday, with the suspect reportedly telling investigators she “wanted to dissect” someone.

Pictures of a bloody hand were posted on an internet forum hours after the weekend killing, with local media saying police were trying to connect the images with the crime scene as more details of the gruesome murder emerged.

The teenager, whose name was not released as she is a minor, was arrested on Sunday on suspicion of murdering her 15-year-old female classmate Aiwa Matsuo, after police discovered the dismembered body on a bed in the suspect's home.

The accused has admitted she strangled the victim before severing her head and left hand, “using tools... and something like a cord”, a police investigator told AFP.

“The victim was found decapitated, with her left hand chopped off,” the investigator said, adding that the murder likely took place on Saturday evening.

The two girls went to the same high school in Sasebo, a city in south-western Japan, police said.

The popular Sports Nippon newspaper said the alleged killer told police she “wanted to dissect” a body. The victim's belly was cut open, it added.

Police discovered tools, including hammers and a saw, at the suspect's apartment, the top-selling Yomiuri newspaper reported.

The paper said the girl's mother died of cancer last year, and she began living on her own after her father remarried about three months ago.

She had only attended class a handful of times since then, Yomiuri reported.

“I wanted to kill someone. I bought tools by myself,” Yomiuri quoted the girl as telling police.

Described as a strong student and athlete, the teenager had previously been involved in trouble at elementary school - reportedly lacing two of her classmates' food with bleaching agents.

“They teased me while I was studying and I got annoyed,” the girl told a teacher after the incident, Yomiuri reported.

A source at the local education board told the paper: “The death of her mother and the father remarrying would have a big impact on her at this stage - but I don't know if this explains what happened.”

The police official declined to confirm the reports, saying: “We are investigating her motive for the crime, and we're not going to disclose other information”. - AFP