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DOUGLAS COUNTY, Colo. — Prosecutors charged a 16-year-old girl as an adult Tuesday, accusing her of conspiracy to commit murder in a plot at Mountain Vista High School in Highlands Ranch.

Prosecutors in court Tuesday afternoon said Sienna Johnson not only planned but took steps to carry out a mass murder at the school. Authorities found out about the threat, and Johnson and another girl were arrested in December.

They said she had maps and schedules to determine where students would be.

They also said she bought a BB gun to practice shooting and experimented with violent acts on her pets.

Prosecutors said she told a detective that if she was released she would continue her violent plans only she would be more secretive.

The other teenage girl arrested in the case appeared in court separately Tuesday morning. She has not been charged.