Sgt. Michael Pooley of the Tempe Police Department said one of the suspects, a 15-year-old boy wanted to kill his grandmother because he was "tired of being in trouble and disciplined by her," KNXV-TV, Phoenix, reported Wednesday.
Police said the teenager enlisted his 17-year-old girlfriend to help in the killing.
The plan was uncovered, police said, when the 15-year-old left his Facebook page open on a family member's computer. The page had messages between the boy and his girlfriend dating back to Thanksgiving.
The teens were arrested Monday night.
Pooley said the girlfriend was the one who pushed the idea and who claimed to know people who could kill the grandmother.
Pooley said,"They said they wanted to make it look like an accident," KPHO-TV, Phoenix, reported. "He suggested a car accident. And other means were discussed. He didn't want her to suffer. And he wanted it done quickly."
The boy told police he and the 17-year-old female planned to carry out the hit after New Year's.
Both juveniles have been charged with conspiracy to commit murder, a felony.
The district attorney has not yet announced if he will try them as adults.
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