CASTLE ROCK, Colo. (CBS4) – The younger suspect in the deadly shooting at STEM School Highlands Ranch told investigators he wanted the students to “suffer from trauma like he has” and “realize the world is a bad place,” according to the probable cause statement released Thursday.

Alec McKinney is 16 but is being charged as an adult in the murder case. McKinney is a female who identifies as male.

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Alec McKinney (credit: Instagram)

The second suspect, 18-year-old Devon Erickson, said McKinney repeatedly threatened to kill him if he didn’t go along with the plan. He also said that McKinney was “super suicidal and that he wanted to get revenge on a lot of people.”

Devon Erickson in Douglas County Court in May. (credit: Joe Amon-Pool/Getty Images)

McKinney and Erickson told investigators they used an ax and a crowbar to break into a gun safe belonging to Erickson’s parents. They armed themselves with four firearms — a Glock 21, a Beretta M9, a Ruger .22 rifle and a Taurus revolver — and ammunition from the safe.

Both suspects admitted using cocaine in Erickson’s home before the May 7 shooting. Erickson said he consumed the cocaine because McKinney threatened him.

According to the probable cause statement, McKinney said he was born female and is still in the pre-op transitioning phase. McKinney told investigators he targeted students he hated — including a specific student who had always made fun of him, “hated him,” called him names and said he was disgusting for trying to be a guy. McKinney said he got Erickson involved in the plan the night before via Snapchat.

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Click here to read the full Probable Cause Narrative.

Erickson indicted several times that he had wanted to stop McKinney and tried to warn certain students about the planned attacked, but investigators say he couldn’t articulate why he never told an adult.

According to the affidavit, McKinney and Erickson went into room 107 with guns in a guitar case and backpack. McKinney shut the door so that it could not be opened from the outside. Erickson said he pulled a gun and said, “Nobody move.” McKinney said Erickson fired the first shot. McKinney admitted firing the revolver and the Glock until they were both empty. Erickson was tackled by a group of students but McKinney was able to grab a third weapon and ran out of the classroom.

A fellow student, 18-year-old Kendrick Castillo, was killed when he tackled Erickson. Eight other students were injured in the shooting.

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Kendrick Castillo (credit: Charles Burroughs)

McKinney said he planned to go outside and kill himself, but didn’t know how to work the safety. He still had the gun when an armed security guard confronted him and took him into custody.