The girl, part of an elite high school chemistry club, reportedly admired British serial killer Graham Young and kept severed animal body parts including a cat's head in her bedroom.
"Mother has been sick since yesterday, having a rash all over her body," the Asahi Shimbun newspaper quoted the girl as writing on August 19 on the online journal, which was kept under a male name.
Another daily newspaper, the Mainichi Shimbun, reported that the blog said on September 12: "Mother is sick today, too. She had been complaining her legs have been out of it for two or three days and she has finally become almost unable to move."
Police found that the dates of the blog matched the illness of her 47-year-old mother, who was in critical condition at a hospital in Shizuoka prefecture southwest of Tokyo, the reports said.
The girl's brother, who thought she was acting suspiciously, reportedly alerted police who examined her computer and discovered the blog.
She is suspected of mixing her mother's food with thallium, a highly toxic but tasteless substance used in the past to kill rodents.
The girl, who kept the blog for more than three months, also wrote of how she duped people around her.
"I got sympathy from my teacher when I tearfully talked about mother. I guess people are cheated more easily than I expected," she wrote, as quoted by the Sankei Shimbun newspaper.
When students were asked in class to name a person that they admired, the girl said that she respected Young, who started to test poisons on his family at age 14. He later poisoned three people to death and died aged 43 in his prison cell in 1990.
Police declined to comment on the details of the reports but said that a 16-year-old girl was arrested on Monday for "trying to kill her own mother with poison".
Her name is not disclosed as she is a minor.
© 2005 Agence France-Presse
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