DENVER — One of two 16-year-old girls accused of plotting to kill classmates and staff at their Colorado high school last month was formally charged as an adult Tuesday with conspiracy to commit first-degree murder, prosecutors said.

Sienna Johnson appeared in Douglas County Court to face allegations that she and another girl planned to attack Mountain Vista High School in the Denver suburb of Highlands Ranch, said District Attorney George Brauchler.

Brauchler told Reuters he decided to charge Johnson as an adult based on evidence seized by police, including a journal in which he said the girl drew a map of the school, made detailed notes about where people congregated and tracked the movements of security personnel.

He said she also posted pictures of other school shootings to her page on the social networking website Tumblr, “took steps to acquire firearms and went target shooting with a pellet gun.” Prosecutors have not said whether either suspect actually obtained a firearm or had access to weapons.

Johnson’s accused accomplice also appeared in court Tuesday, but a decision on what charges, if any, she could face was postponed pending the completion of a mental-health examination, Brauchler said.

The second girl has not been publicly named because prosecutors have not decided whether she will face adult charges.

Both girls had been held without bail since their arrests on Dec. 12 after investigators said they planned to harm students and staff at the 2,000-student school.

Douglas County District Court Judge Paul King set a $1 million bond for Johnson during Tuesday’s hearing.

Police took the pair into custody after two anonymous tipsters warned them the girls were planning an attack, said Sergeant Lori Bronner, spokeswoman for the Douglas County Sheriff’s Office.

Arrest warrant affidavits and other documents related to the case were placed under court seal, with a hearing set for next week on whether they should be released, a court spokeswoman said.

Colorado has been the scene of at least four firearms attacks on schools over the past two decades, the deadliest being the 1999 massacre at Columbine High School, where two students fatally shot a teacher and 12 classmates before killing themselves.

Brauchler said Johnson also mentioned the movie “Natural Born Killers” in her journals, which also was referenced by the Columbine shooters in their written and video manifestos.