Second teenager pleads guilty in planned attack on high school 06/07/2017 12:54 pm PDT Comments KDVR By Chuck Hickey, KDVR DOUGLAS COUNTY, Colo. -- (KDVR) -- A second teenager accused of plotting to kill students and staff members at Mountain Vista High School entered a plea agreement Wednesday morning. Sienna Johnson pleaded guilty to an adult count felony menacing and a juvenile count of conspiracy to commit murder related to the plan to attack the Highlands Ranch school in December 2015. She will spend three years in the juvenile detention system and four years on probation. It's a similar deal reached by co-conspirator Brooke Higgins. Both girls are now 17 and were originally charged as adults. Higgins pleaded guilty in December to one count of conspiracy to commit murder and murder solicitation and was sentenced in February to three years in a youth detention center and four years of probation. The attack plot was thwarted after a student tipped off the Douglas County Sheriff's Office According to an affidavit, Higgins and Johnson had a detailed map of the school and spent time online looking for weapons. The affidavit said Higgins wished she could've “done Columbine with Eric and Dylan,” a reference to the 1999 Columbine High School massacre. FULL STORY: Second teenager pleads guilty in planned attack on Mountain Vista High School - KDVR #Colorado #KDVR Related Stories Teen girl charged as adult in alleged mass-murder plot at high school Waiting for your permission to load facebook comments.