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Mourners react as they visit a makeshift memorial outside of Oxford High School on December 01, 2021 in Oxford, Michigan. Image Credit: AFP

Michigan prosecutors on Wednesday charged the teenage suspect in this year’s deadliest US school shooting with first-degree murder and other charges as investigators sought to determine what prompted the spree that killed four students and wounded seven other people.

Ethan Crumbley, a 15-year-old sophomore at a high school in Oxford, Michigan, about 65km north of Detroit, was charged with a slew of criminal counts, Oakland County Prosecuting Attorney Karen McDonald said.

"I am absolutely sure after reviewing evidence that it isn't even a close call,” she said at a briefing. “It was absolutely premeditated."

In addition to four counts of first-degree murder, Crumbley faces one count of terrorism causing death, seven counts of assault with intent to murder and 12 counts of possession of a firearm in the commission of a felony, she said.

Crumbley, who is being charged as an adult, was arraigned later on Wednesday. McDonald said it was possible more charges could be issued very soon.

The shooting spree was the deadliest on US school property this year, according to Education Week. It was the latest in a decades-long string of deadly American school shootings that will likely fuel debates about gun control and mental health care.

Crumbley opened fire at Oxford High School with a semi-automatic handgun — which his father had purchased four days earlier — after emerging from a restroom shortly before 1pm EST (10pm UAE) on Tuesday, authorities said.

Oakland County Sheriff Michael Bouchard said Crumbley did not appear to be targeting any specific people during the shooting spree.

Three students hit by gunfire — Tate Myre, 16, Hanna St. Julian, 14, and Madisyn Baldwin, 17, — died on Tuesday, the Sheriff’s Office said. The fourth, 17-year-old Justin Shilling, died on Wednesday.

Of the six students and one teacher who were wounded, three students remained hospitalized late Wednesday afternoon, including a 17-year-old girl in critical condition with a gunshot wound to the chest, the Sheriff’s Office said. Four others, including the teacher, have been discharged.

Bouchard told a briefing that investigators have yet to determine a motive for the shooting, adding that there was no evidence Crumbley had been bullied.

But he said school officials had contact with Crumbley the day before the shooting and another meeting with him and his parents on the morning of the shooting for behavior in the classroom that they felt was concerning.

“The content of that meeting, obviously, is part of the investigation, but we did not learn of that meeting nor the content of that meeting until after the shooting and during this investigation,” he said.

More than 50,000 people had signed an online petition as of Wednesday morning to rename the school’s stadium after Myre, who played on Oxford High’s football team, saying he tried to disarm the shooter.

“Tate is not just a hero to his fellow students at Oxford High School but a legend, his act of bravery should be remembered forever and passed down through generations,” the petition on Change.org said.

Bouchard said earlier that investigators were poring over writings of the shooter they obtained in the middle of the night that contain “some of his thoughts.” They were also watching surveillance videos of the incident.

Bouchard credited swift action by his deputies for preventing greater loss of life, saying they arrived on the scene within minutes and moved straight toward the sound of gunshots. Crumbley, who did not resist, was disarmed and taken into custody minutes after the shooting began, he said.