OXFORD, Mich. (AP) — Officials with Michigan’s Oxford High School should have conducted a threat assessment into Ethan Crumbley’s behavior prior to a shooting that left four students dead and others wounded, an independent investigation concluded.

Crumbley’s conduct included viewing bullets, watching violent video on his cellphone during class, and writing statements like “blood everywhere,” a more than 500-page report said. That suggested “not suicide, but homicide,” according to the report released Monday, a month shy of the two-year anniversary of the Nov. 30, 2021, shooting.

Crumbley, 17, pleaded guilty a year ago to first-degree murder and terrorism charges. He is expected to be sentenced Dec. 8 in Oakland County Circuit Court in Pontiac.

The teen and his parents met with school staff the day of the shooting after a teacher noticed violent drawings. But no one checked his backpack and he was allowed to stay. The gun used in the shooting was in the backpack.

The shooter also kept a journal and wrote about his desire to watch students suffer and the likelihood that he would spend his life in prison. He made a video with his phone on the eve of shooting, declaring what he would do the next day.

The independent investigation into what transpired before the shooting, the day of the shooting and in its aftermath was started by the Oxford Community Schools Board. Neither the school board nor the school district were engaged in the investigation, the report said.

Killed were Madisyn Baldwin, Tate Myre, Hana St. Juliana and Justin Shilling at Oxford High, about 40 miles (60 kilometers) north of Detroit. Six students and a teacher were also wounded.

“Our review confirmed that there were breakdowns in implementation and execution of the district’s threat assessment and suicide intervention policies and guidelines,” the report said. “Missteps at each level throughout the district — from the board, to the superintendent, to the OHS administration, to staff — snowballed to create a situation where a student’s communications and conduct should have triggered a threat assessment and suicide intervention on Nov. 30, but did not.”

“None of these mistakes were intentional. But costly mistakes they were,” it continued.

It also said the school should have sent the teen home with his parents following a meeting with school officials the day of the shooting.

James and Jennifer Crumbley are charged with involuntary manslaughter. They are accused of making a gun accessible at home and ignoring their son’s mental health.

Prosecutors are seeking life in prison without the possibility of parole for Ethan Crumbly, who was 15 when the shooting happened. The judge also has the option of sentencing him to a shorter term of somewhere from 25 years to 40 years.