CASTLE ROCK, COLORADO: The younger of two students who were charged as adults in connection to the May 2019 shooting at the STEM School Highlands Ranch in Douglas County, Colorado, that killed one has pleaded guilty to his charges.

Alec McKinney, 16, pleaded guilty to 17 felonies, including a first-degree murder charge, this past Friday, February 7, under an agreement that gives him a chance to be paroled after serving around 25 years in prison, according to the Associated Press.

McKinney and his co-defendant, 19-year-old Devon Erickson, stormed the school on May 7, 2019, after arming themselves with three handguns and a .22-caliber rifle stolen from a gun safe owned by the latter's parents. Both had also reportedly taken cocaine.

The subsequent attack injured eight and killed 18-year-old Kendrick Castillo, who was one among the students who rushed Erickson after authorities said he pulled out a gun in his classroom. Both McKinney and Erickson were later successfully apprehended.

McKinney was born a female but was transitioning to male at the time of the shooting and confessed to targeting specific students who bullied him and called him "disgusting" for going through the process, according to an arrest affidavit. 

He said some of his classmates made fun of him, and repeatedly referring to him as a "she" despite him identifying as a male.

The teen told police he "wanted the kids at the school to experience bad things, have to suffer from trauma" like he had to in his life and that he "wanted everyone in that school to suffer and realize that the world is a bad place".

His defense attorneys had brought up that trauma during an unsuccessful bid to have the case transferred to juvenile court and said he had a troubled childhood where he witnessed domestic violence by his father Jose Evis Quintana against McKinney's mother.

It was revealed that Quintana was a serial felon and illegal immigrant from Mexico and was once jailed for 15 months for domestic violence against McKinney's mother and "menacing with a weapon".

While McKinney's guilty plea this past week did come in an adult court, his juvenile status means he faces less time in prison than if he had committed the crimes as an adult.

The minimum sentence for a juvenile convicted of first-degree murder is life in prison with the possibility of parole after 40 years, though a 2016 state law that applies to inmates sentenced when they were juveniles means he will be eligible for the same in 20 or 25 years.

This would apply even if the judge decides that he serve his sentences for the other 16 counts on top of the life sentence for first-degree murder. However, it will involve McKinney having to apply to enter a three-year prison program after serving 25 years, or earlier if they have earned prison credits.

McKinney will be sentenced on May 18.

Erickson has pleaded not guilty to the same charges McKinney faced in the shooting and his lawyers have portrayed McKinney as the ringleader who pressured Erickson to participate.