The perpetrator in the STEM School Highlands Ranch shooting pleaded not guilty at his arraignment in Douglas County District Court on Thursday (January 2), according to The Denver Channel.

The tragedy occurred on May 7, 2019, when Devon Erickson, 18, and Alec McKinney, 16, opened fire inside of the school at 1:53 p.m.

Erickson was eventually tackled by three other students, including 18-year-old Kendrick Castillo, whom he fatally shot in the chest. Eight other students were injured in the attack.

The two suspects were taken separately into custody.

Erickson was charged with 44 felony counts, including first-degree murder, attempted first-degree murder, and multiple misdemeanors. He will be tried as an adult.

His attorneys are arguing that he was forced into the shooting by McKinney.

They have also entered a notice to present evidence of Erickson's mental state, which will require Erickson to be evaluated by a psychiatrist from the county jail or a state mental health facility.

McKinney is facing murder charges as well and will also be tried as an adult.