CNN  — 

Days after a 15-year-old student allegedly killed four classmates at a Michigan high school, the district’s superintendent has requested an independent investigation into the incident – the deadliest school shooting at a US K-12 campus since May 2018.

Ethan Crumbley is accused of carrying out Tuesday’s shooting at Oxford High School in suburban Detroit and faces a slew of charges as an adult, including terrorism and first-degree murder, among others. His lawyer entered a not-guilty plea during his arraignment Wednesday.

His parents, James and Jennifer Crumbley, were also each charged with four counts of involuntary manslaughter in connection with their son’s alleged actions.

The shooting claimed the lives of Madisyn Baldwin, 17; Tate Myre, 16; Hana St. Juliana, 14; and Justin Shilling, 17, authorities said.

On Saturday, Tim Throne, who heads the Oxford Community Schools district, detailed in a letter the school’s account of several key events leading up to the shooting.

“It’s critically important to the victims, our staff and our entire community that a full and transparent accounting be made. To that end, I’ve asked for a third-party investigation be conducted so we leave no stone unturned, including any and all interaction the student had with staff and students,” Throne wrote in the letter addressed to the Oxford community.

The day before the shooting

On Monday, a teacher saw the suspect looking at photos of ammunition on his cell phone during class, which prompted a meeting with a counselor and another staff member. During that discussion, the student told them that he and his mother had recently gone to a shooting range and that “shooting sports are a family hobby,” Throne wrote in the letter.

The school tried to reach the student’s mother that day, but didn’t hear back until the following day when his parents confirmed the student’s story, Throne said.

After school officials reached out to Jennifer Crumbley regarding her son searching the web for ammunition, she texted him saying, “LOL I’m not mad at you. You have to learn not to get caught,” prosecutors have said.

The morning of the shooting

Then on Tuesday – the day of the shooting – a teacher alerted school counselors and the Dean of students to “concerning drawings and written statements” that the student had created, according to the letter. He was “immediately removed from the classroom” and taken to a guidance counselor’s office, Throne explains.

The student told a school counselor that “the drawing was part of a video game he was designing and informed counselors that he planned to pursue video game design as a career,” Throne said.

Following that discussion, the student stayed in the office for an hour and half as school staff called his parents and waited for them to arrive to the school, the letter noted. While waiting the student said he was concerned about missing his homework assignments and “requested his science homework, which he then worked on while in the office,” the letter said.

“At no time did counselors believe the stud