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CASTLE ROCK, Colo. (KDVR) — Devon Erickson and Alec McKinney, who carried out the 2019 STEM School Highlands Ranch shooting, planned to “execute” targeted students in a locked classroom, with the plan to end with Erickson shooting his co-conspirator dead, McKinney testified Tuesday.

McKinney, who was 16 at the time of the shooting and is already sentenced to life in prison plus 38 years, testified in Erickson’s continuing trial in Douglas County court. Erickson’s defense has focused on McKinney as the manipulator in the crime.

McKinney said their brief friendship started months before the shooting, around January 2019, and progressed from chatting on social media and smoking weed on lunch breaks to planning the school shooting.

“I remember it starting as a late night conversation we had, where we were messaging back and forth on Snapchat, and we got on the topic of, ‘What’s the most messed up thing on your bucket list?'” McKinney testified.

“I remember saying I wanted to try acid, and Devon had said that he wanted to get away with killing someone,” he said.

The conversations turned more serious, and the two discussed various ways to carry out a killing. Erickson at one point suggested inviting people to a party in a cabin and killing them there, McKinney testified.

As they discussed the plan, they read about the Columbine shooting. And weeks before the STEM shooting, they heard the news about Sol Pais, who traveled from Florida to Colorado and bought a gun, prompting school closures in the Denver metro for fear of what she planned to carry out while her whereabouts were unknown.

McKinney said they felt they “related” to Pais, who documented a “detachment from reality” and loneliness on a personal website publicized after her high-profile death. Pais ultimately used the gun to take her own life along a mountain trail.

The plan evolved until a couple of weeks before the shooting, and it ultimately narrowed down to the specific classroom, No. 107, where Erickson opened fire that day.

That classroom included specific people Erickson wanted to “unleash” on, said McKinney, who named a list of those people in court. McKinney said he also had a list of people to target, including one student who said negative things about transgender people like McKinney.

McKinney said he planned to die that day. The two planned to blame the shooting on McKinney, because “I didn’t really care about my life, nor what was really said about me after I was gone, so it would be easier if May the 7 would be the day that I would, um” — he paused with emotion — “die.”

“He was going to shoot me to make it look like he attempted to save everyone, and ultimately, he would come off as the hero,” McKinney said.

McKinney’s testimony continues Wednesday.

Watch the live stream of the trial on FOX31 NOW.