The girl, from Colarado, has been detained by authorities for trying to kill her mother twice.
On both occasions the girl attempted to poison her mother by adding bleach to her drinks, police reported.
In the first suspicious incident the mother felt sick after drinking a smoothie, according to Boulder Country Sheriff’s Office.
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The office added that while the mother could smell bleach, the woman assumed her daughter hadn’t rinsed it out well enough.
After the woman noticed the smell of bleach the second time she confronted her daughter.
The teenager reportedly told her mother she was trying to kill her because she took away her iPhone.
A few days after these incidents the woman was at Boulder Community Hospital where she contacted the police.
The mother is no longer in hospital and the 12-year-old was detained, on two counts, of attempted first-degree murder, on Friday.
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Sgt. Bill Crist of the sheriff’s office said: “She had pre-planned this a couple of different times.”
The investigation is still on-going.