A girl made it easy for police, telling them “I did it all by myself.”
Cops found the body of 15-year-old Aiwa Matsuo in her friend’s apartment Monday.
Her head and left hand were cut off.
The two had been friends for years.
The Japan Times reports police believe the girl, who has yet to be named, killed her friend by hitting her in the head and then wrapping a cord around her neck.
"We are not aware of any trouble between the two," a worker at the school the girls attended told the paper.
However, authorities are studying messages left on an Internet site called 2Channel.
"I've tried to warm (the body) as it got cold, but it doesn’t warm up,” one said.
Another asks, “everyone, (do you want to know) what color the brain is?" It continued, “I’ll take good care of the brain and the spinal cord, putting them in a solution."
Police say the girl suspected has shown no emotion.