CASTLE ROCK, Colo. — One of two 16-year-old girls accused of conspiring to kill students at their suburban Denver high school has been charged as an adult after prosecutors say she bought a BB gun for target practice and mapped plans for an attack.

Prosecutors on Tuesday charged Sienna Johnson with conspiracy to commit murder and asked that she not be allowed to return to Mountain Vista High School in the affluent suburb of Highlands Ranch. Defense attorneys requested a hearing to transfer the case back to juvenile court.

A judge set her bond at $1 million. Prosecutors said Johnson showed no remorse and told investigators that if she were released she would try again and better hide her plans.

The girls were arrested after police were alerted to their plans on Dec. 12 via a text-a-tip program developed for schools.

Prosecutors are awaiting the results of a mental health exam and other evidence before deciding whether to charge the second girl as an adult. She faces another hearing on Jan. 14.

Her parents declined to comment after Tuesday’s hearing, and Johnson’s parents could not be located for comment.

In addition to buying the BB gun, authorities said Johnson had harmed a pet and kept a journal where she drew disturbing images and wrote about the 1994 film “Natural Born Killers,” which profiled young serial killers. She obtained maps of the school and the schedule of a school resource officer, prosecutors said.

An attorney for one of the girls argued in court filings that the case hinges on entries in a journal that police illegally seized. The girl was keeping the journal at the urging of a therapist, attorney Dagna Van Der Jagt said in the motion, The Denver Post reported.

“There is no direct evidence in this case of the defendant’s possession of any weapons, bombs, or incendiary devices,” Van Der Jagt wrote in the motion opposing a request by media outlets to unseal documents in the case. The judge did not immediately rule on whether to release documents in the case.

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