One of two teenagers accused of planning a Columbine-style attack at Mountain Vista High School in late 2015 pleaded guilty Wednesday to conspiracy to commit murder and felony menacing.

Sienna Johnson, 17, also pleaded guilty to being a “violent juvenile offender” – which makes her potential sentence more severe.

Johnson was originally charged with two counts of conspiracy to commit first-degree murder.The conspiracy and violent offender charges were filed in juvenile court. The felony menacing charge was filed in adult court.

Her plea agreement called for a five-year sentence to the Department of Youth Corrections followed by four years on probation, during which time she will be closely monitored.

That agreement was approved by District Judge Paul A. King during Wednesday's hearing.Sentencing is set for 9 a.m. on Aug. 11 and is expected to include testimony from a doctor who evaluated Johnson.

Dressed in a gray polo shirt, with her dark hair pulled into a bun, Johnson patiently answered a series of questions the judge asked to make sure she understood the plea and the ramifications of that.

“Guilty,” Johnson said when the judge asked her for a plea on each on the felony menacing charge, the first one taken up by the court. She later pleaded guilty to the other two charges.

Johnson's sentence will mean she will have an adult felony conviction on her record for the rest of her life.

While she is on probation, she will be prohibited from using controlled substances – including medical marijuana even if she obtains a prescription – and can't possess any weapons, including knives. Her computer use will also be monitored when she is on probation.

“At the end of the day I have great hope that both of these girls end up growing up to be responsible, law-abiding members of society that pose a risk to no one,” District Attorney George Brauchler said after the hearing. “We can’t say that today and so that’s why we put this particular sentence in place.”

Johnson’s father and attorneys left the hearing without commenting.

Johnson and a classmate, Brooke Higgins, were accused of planning to carry out the attack on Dec. 17, 2015 – the last day of classes before the holiday break at the Douglas County school.

A tip led to their arrests.

Higgins, also 17, pleaded guilty last December to solicitation of first-degree murder and conspiracy to commit first-degree murder. She was sentenced to three years in the Department of Youth Corrections on the murder solicitation charge. Once that is complete, she is expected to begin what is known as a deferred judgment on the second felony charge, conspiracy to commit first-degree murder. That sentence would require her to stay out of trouble for four years.

There are some important distinctions between the sentences for the two girls.

Higgins, for instance, will have the adult felony conviction wiped off her record if she successfully completes probation – but could face as much as 24 years in prison if she doesn’t.

Johnson, on the other hand, will always have an adult felony conviction on her record – but if she fails to successfully complete probation she faces a maximum of six years behind bars.

During Higgins' sentencing hearing earlier this year, prosecutors recited a Snapchat conversation between the two: “We have to get guns, get good at shooting them, plan out how we will get the most people. Then do it. Let's become best friends. Columbine part 2.”

A previously released arrest warrant affidavit in the case laid out the chilling details of the alleged plot.

According to the document, one source told investigators the attack was planned for mid-December because it was close to Christmas and it “would be the most ‘traumatic’ for everyone.”

Higgins also wrote that “a day did not go by that she did not think about killing herself or the people around her” and that “she wished she had done Columbine” with the two seniors who murdered a dozen students and a teacher at that Jefferson County school on April 20, 1999. “Everything” in a journal kept by one of the Columbine killers “made sense to her,” according to the affidavit.

According to the affidavit, the plot came to light after someone using a system known as Text-A-Tip alerted authorities to the plans. On Dec. 9, 2015, Johnson was taken to Children’s Hospital Colorado and placed on a mental health hold. A few days later, according to the affidavit, Johnson’s mother called authorities after discovering journals in two spiral notebooks that detailed the planned attack at Mountain Vista, a school of roughly 2,200 students.

A psychiatric nurse at the hospital later told authorities that Johnson told her she and Higgins went online in an effort to buy guns and that “there was a pact between the two girls to carry out the shooting no matter if one side was unavailable to participate,” according to the affidavit.

Johnson also told the nurse that “her plan was to kill her mother and sister prior to committing the shooting at the school” and that the two girls planned to kill themselves after attacking Mountain Vista.

Authorities then went to Higgins’ home, where they discovered a purple journal that detailed the plan.

In the journal, according to the affidavit, Higgins wrote that she’d been using cocaine, that she was sad, and “she was tired of everything and the only path with hope is one with evil, chaos and destruction.”

In another entry, she wrote that “many people deserve to be shot,” that she “needed to get a gun” and that “it was good she didn’t have one because she’d be dead or others would be,” according to the affidavit.

Although the girls initially planned the attack for Dec. 17, 2015, they later moved the plan to January of 2016 because they had been unable to get their hands on guns.

Johnson, in the meantime, told a detective that she thought the Columbine killers were “gods” and had begun practicing with a BB gun “in order to get used to holding a gun and the feeling of a gun,” according to the affidavit.

Higgins and Johnson were both originally charged as adults with conspiracy to commit first-degree murder under extreme indifference and conspiracy to commit first-degree murder after deliberation.

Defense attorneys had portrayed the allegations against the girls as the equivalent of “thought” crimes. But it was clear in various court proceedings that prosecutors viewed the plot as much more serious, and the affidavit shows in one case multiple parallels to Columbine.

Contact 9NEWS reporter Kevin Vaughan with tips about this or any story: or 303-871-1862.