Ethan Crumbley, who is accused of killing four fellow students at a Michigan high school, will be tried as an adult and faces murder, assault and weapons charges.
He will also face one count of terrorism causing death, a rare charge for a school shooting.
The events unfolded Tuesday at Oxford High School when, law enforcement officials say, the 15-year-old shot at people in a school hallway, firing more than 30 shots at people and through classroom doors. Three people died Tuesday and another passed away at a hospital Wednesday.
Seven others — six students and a teacher — were wounded, Oakland County Sheriff Michael Bouchard said.
The county's top prosecutor addressed the terrorism charge.
"There is no playbook about how to prosecute a school shooting and candidly, I wish I'd never even had — it didn't occur so I wouldn't have to consider it, but when we sat down, I wanted to make sure all of the victims were represented in the charges that we filed against this individual," Oakland County Prosecutor Karen McDonald told CNN. "If that's not terrorism, I don't know what is."
She said there is a lot of digital evidence in the case — video and things on social media.
"But you probably don't even need to see that to know how terrifying it is to be in close proximity of another student shooting and killing fellow students. I mean, it's terror," she added.
"Like every other child that was in that building, and I address that about the terrorism charge, we must have an appropriate consequence that speaks for the victims that were not killed or injured but also, they were affected, how do they go back to school?"
She said many students can't eat or sleep.
"Their parents are sleeping next to them and we shouldn't ignore that," she told CNN. "There are obviously four children who were murdered and many others injured but over 1,000 were also victimized as well."
At Crumbley's arraignment Wednesday, prosecutor Marc Keast said Crumbley came out of a school bathroom and started firing. Crumbley walked down the hall at a "methodical pace" and fired more shots.
This continued for another four or five minutes and he went to another bathroom, Keast said. When deputies arrived, Crumbley put the gun down and surrendered, the prosecutor said.
The judge entered a plea of not guilty per his attorney's request.
Michigan law defines an act of terrorism as a "willful and deliberate act that is all of the following:"
"An act that would be a violent felony under the laws of this state, whether or not committed in this state.
"An act that the person knows or has reason to know is dangerous to human life.
"An act that is intended to intimidate or coerce a civilian population or influence or affect the conduct of government or a unit of government through intimidation or coercion."
The criminal complaint against Crumbley refers to the third condition and says the act was committed against the Oxford High School community.
Charging an accused school shooter with terrorism is rare. In 2018, an Ocala, Florida student who shot through a door and wounded another student, was charged with terrorism and later pleaded no contest to that count and other charges.
That incident occurred two months after gunman Nikolas Cruz shot more than 30 people as he moved for more than 10 minutes through Marjory Stoneman Douglas High School in Parkland, Florida.
Cruz was charged with 34 counts of premeditated murder and attempted murder. He did not face a terrorism charge. He recently pleaded guilty and is awaiting sentencing.
CNN's Shimon Prokupecz reported from Oxford and CNN's Steve Almasy reported and wrote in Atlanta. CNN's Carolyn Sung in Michigan and Kristina Squeglia contributed to this report.