A 16-year-old girl has been charged as an adult with conspiracy to commit murder in connection with a plot to attack the Colorado high school she attended.

KUSA-TV reports that Sienna Johnson faces two counts of conspiracy to commit first-degree murder. She and another female student were arrested in December after authorities uncovered an alleged plot to carry out an attack at Mountain Vista High School In Highlands Ranch, Colo.

Bond for Johnson was set at $1 million.

Prosecutors say the girl had compiled detailed maps of the school showing student movement and had a schedule for the police officer assigned there. Evidence showed that the girl made references to other school shootings on a social media account and also mentioned Natural Born Killers, a film that was referenced repeatedly by the two students who carried out the deadly assault on Columbine High School in 1999.

Johnson also allegedly told investigators that if she were released from custody, she would try again to carry out her plans, but would be more discreet.

The second girl arrested in the case has not been formally charged while she awaits a psychiatric evaluation.