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A combined SWAT team waits outside the middle school entrance at STEM School Highlands Ranch after a shooting that killed one and wounded eight students. (John Leyba, Special to The Colorado Sun)

The two teens accused in the deadly shooting at STEM School Highlands Ranch told authorities the attack was carried out to exact revenge on some of their peers and that they got the weapons they used from a gun safe belonging to one of their parents.

One of the alleged shooters told investigators that the pair entered the Douglas County school through a door where they knew the rifle and handguns they were carrying would not be detected.

That’s according to court documents unsealed on Thursday afternoon, that for the first time provide details about the May 7 attack that left 18-year-old Kendrick Castillo dead and eight more students wounded. The documents reveal that the shooting had apparently been in the works for weeks.

MORE: Read the court documents.

A probable cause statement for the arrest of 18-year-old Devon Erickson, one of the alleged shooters, also shows that a private security guard at the school fired two rounds at a responding Douglas County sheriff’s deputy in the chaos of the response.

Both shots missed the officer, but one wounded a female student who was apparently in a classroom, the document indicates.

Erickson and 16-year-old Alec McKinney are accused of murder and attempted murder in the shooting. McKinney has been charged as an adult, but his attorneys are seeking to return the case to juvenile court. Erickson faces a total of 48 counts.

The court documents indicate McKinney, whose name is mostly redacted from the report, told investigators the pair entered a classroom where “all of the kids they hated would be.” Erickson shouted “nobody move” before the two opened fire.

Erickson was tackled by other students — including Castillo, according to reports. McKinney also was rushed by people in the classroom, but was able to run out before being apprehended soon after.

Paramedics examine a STEM Highlands Ranch school teacher for injuries after a shooting that wounded at least eight students on Tuesday, May 7, 2019, in Highlands Ranch, Colorado. The alleged shooters, two students at the school, are in custody. (John Leyba, Special to The Colorado Sun)

McKinney told investigators that he planned to die by suicide after leaving the school, but didn’t know how to release the safety mechanism on the handgun he fled the classroom with.

Erickson, according to the probable cause statement, blamed the attack on McKinney, saying the younger teen threatened to kill him. He said he had at times thought of revealing their plans to authorities.

“Devon repeatedly claimed he was going to stop (redacted) but couldn’t articulate how or why he never told an adult,” the document says, apparently referring to McKinney.

Kendrick Castillo, 18, died in the May 7 shooting. (Colorado State Patrol handout)

In interviews with detectives, McKinney said he had been transitioning from female to male over a three-year-period and had been bullied by classmates who called him disgusting.

McKinney — whose birth name is Maya — told investigators that he had specific targets at the school in mind, but “wanted everyone in that school to suffer from trauma like he has in his life and to realize that the world is a bad place.”

The probable cause statement says Erickson and McKinney had only known each other for a half a year before the shooting, but doesn’t indicate how they met beyond being classmates or detail what their actions were in the days and weeks leading up to the attack.

According to the document, the pair decided to carry out the shooting during a Snapchat conversation the night before.

“Devon told (McKinney) a few weeks prior that his parents owned guns and they were locked in a safe,” the probable cause statement says.

The two went to school in the morning before the shooting and then decided to meet at Erickson’s home during lunch.

MORE: STEM School has until June 29 to allay concerns and secure extended charter reauthorization

In interviews with detectives, McKinney said he threatened Erickson with an ax at the home, which motivated Erickson to smash into his family’s gun safe with the tool.

The two then pried open the safe with a crowbar and took out three handguns, a rifle and ammunition.

They used cocaine and spray painted Erickson’s mom’s vehicle in the garage and set it on fire, according to the warrant, before heading to their school to attack their classmates.

Students from STEM School Highlands Ranch walk from their school entrance, with their hands on their heads, after a shooting that killed one student and wounded eight others on May 7, 2019, in Highlands Ranch, Colorado. (John Leyba, Special to The Colorado Sun)

Then the pair went back to STEM School, entering through a door at the middle school where McKinney told investigators they knew the guns being carried in a backpack and guitar case would go undetected.

The security guard who wounded a student, whose name is redacted from the court documents, told investigators that he fired after seeing a “muzzle come around the corner.” He has been credited with detaining McKinney during the attack.

A special prosecutor — the 4th Judicial District Attorney’s Office in Colorado Springs — has been reviewing the guard’s actions to see if he should be charged with any crime.  

STEM School did not have a school resource officer because of a disagreement with the Douglas County Sheriff’s Office.

Senior editor Dana Coffield contributed to this report.

Jesse Paul is a Denver-based political reporter and editor at The Colorado Sun, covering the state legislature, Congress and local politics. He is the author of The Unaffiliated newsletter and also occasionally fills in on breaking news coverage....