The parents of a 15-year-old boy who shot dead four students at a high school in the US state of Michigan with a gun bought for him by his father just days earlier were charged with involuntary manslaughter on Friday.

Oakland County prosecutor Karen McDonald, in a rare move by law enforcement, announced that James and Jennifer Crumbley, the parents of Ethan Crumbley, each face four counts of involuntary manslaughter.

"These charges are intended to hold the individuals who contributed to this tragedy accountable and also send the message that gun owners have a responsibility," McDonald said at a press conference.

"While the shooter was the one who entered the high school and pulled the trigger, there are other individuals who contributed to the events on November 30 and it's my intention to hold them accountable as well," she said.

Oakland County Sheriff Michael Bouchard said the whereabouts of the parents was not currently known and they were considered fugitives.

"We'll have them in custody soon," Bouchard said. "They cannot run from their part in this tragedy."

Four students, aged 14 to 17, were killed in the shooting at Oxford High School north of Detroit and six more were wounded, along with a teacher.

Ethan Crumbley has been charged as an adult with state murder and terror charges.

While school shootings carried out by teens occur frequently in the United States, it is unusual for parents to face charges.

James Crumbley bought the 9mm Sig Sauer semi-automatic handgun used by his son four days before the shooting.

Ethan Crumbley was with his father at the time of the purchase at a local firearms store and the teenager posted a picture of the gun on his Instagram account, writing "just got my new beauty today" along with a heart emoji.

According to police, Ethan Crumbley recorded a video on his cell phone the night before the attack saying he was planning a shooting at the school the next day, but it was not posted online.

That same day, a teacher at the school had observed Ethan Crumbley searching for ammunition on his cell phone during class and reported it to school officials.

His mother was contacted by the school but did not respond to voicemail or email messages.

McDonald said Jennifer Crumbley did exchange a text message about the incident with her son that day, writing: "lol I'm not mad at you. You have to learn not to get caught."

'Don't do it'

The parents were summoned to the school on the day of the shooting itself after a teacher was "alarmed" by a note she found on Ethan Crumbley's desk, McDonald said.

It featured a drawing of a gun and the words "The thoughts won't stop. Help me."

It also had a picture of a bullet, a person who had been shot and the words "my life is useless" and "the world is dead," she said.

The parents were shown the drawing at a meeting with school officials and advised that they needed to get the boy into counselling within 48 hours.

McDonald said they resisted taking their son home and he returned to class. He later entered a bathroom, emerged with the gun, which he had concealed in his backpack, and opened fire.

"The notion that a parent could read those words and also know that their son had access to a deadly weapon that they gave him is unconscionable and I think it's criminal," McDonald said.

"I am angry," she said. "I'm angry as a mother. I'm angry as the prosecutor. I'm angry as a person that lives in this county.

"We need to do better in this country," she said. "We need to say enough is enough for our kids, our teachers, parents, for all of us in this community and the communities across this nation."

Ethan Crumbley fired off at least 30 rounds, reloading with a fresh ammunition magazine as fellow students fled.

Students and teachers barricaded themselves in classrooms, as they had been taught to do in drills, and some escaped the school through windows.

McDonald said that Jennifer Crumbley, when she heard about the shooting, had texted her son, saying, "Ethan don't do it."

James Crumbley, when he heard the news, drove home and then called the emergency line 9-1-1 to report that a gun was missing from his house and that he believed his son may be the shooter, McDonald said.
Involuntary manslaughter carries a sentence of up to 15 years in prison.