Prosecutors on Wednesday charged Oxford High School student Ethan Crumbley, 15, as an adult.
He has pleaded not guilty. Police are yet to identify a motive in the attack.
The victims were named as Tate Myre, 16, Madisyn Baldwin, 17, Hana St Juliana, 14, and Justin Shilling, 17.
Announcing the charges on Wednesday, Oakland County prosecutor Karen McDonald said her office had “a mountain of digital evidence” to show the suspect had planned the attack “well before the incident”.
One such instance is a video of the suspect the night before the shooting, in which he discusses killing students.
“This isn’t even a close call,” she said. “This was not an impulsive act.”
She added that it was “absolutely premeditated”.
Ms McDonald said the teen will face one count of terrorism, four counts of first-degree murder, seven counts of assault with intent to murder, and 12 counts of possession of a firearm, with further charges likely as the investigation unfolds.
At a later news conference, Oakland County Sheriff Michael Bouchard said charging the suspect as an adult was “the most appropriate” action.
“I agree with holding this individual wholly and completely responsible for the deplorable, tragic event that occurred by his choice,” he said.
The suspect will be transferred from a juvenile detention centre to the local county jail, where he will be held without bond.
The teenager has refused to speak with investigators on the instructions of his family.
The sheriff stressed that the teenager had never been on the radar of law enforcement or the school. He also added there was no evidence the suspect had been bullied by his peers.
“There is nothing that he could have faced that would warrant senseless, absolutely brutal violence on other kids,” said Mr Bouchard.
He noted that the suspect’s parents had been called to the school hours before the shooting “for behaviour in the classroom that was concerning”.
Investigators have found the suspect loaded his firearm – purchased by his father on Friday – in the bathroom and then walked through the hallway firing more than 30 shots from the weapon.
Mr Bouchard said the suspect appeared to fire indiscriminately during the attack at the school of around 1,700 students.
Tate Myre reportedly tried to disarm the shooter, and more than 75,000 people have signed a petition calling for the school to rename its football stadium after Tate.
The 16-year-old, who played on the football team, died in a police officer’s car on the way to hospital.
Madisyn and Hana also died on Tuesday, while Justin was confirmed dead on Wednesday.
Among those admitted to hospital, a 17-year-old female student remains in critical condition with a gunshot wound to the chest.
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