US authorities are searching for the parents of the Michigan teenager accused of murdering four students at his high school.

It happened hours after the couple was charged with involuntary manslaughter for buying their son the weapon as a Christmas gift and ignoring warning signs as late as the day of the shooting.

A fugitive warrant has been issued for James and Jennifer Crumbley, who had been scheduled for arraignment on four counts of manslaughter each later today.

It comes three days after authorities say their 15-year-old son, Ethan, carried out the deadliest US school shooting of 2021.

Oakland County Sheriff Michael Bouchard told CNN police were searching for the Crumbleys after the couple's attorney told his office that they have stopped responding to messages.

"If they think they're going to get away, they're not," Mr Bouchard said, adding that a "host" of detectives, as well as the FBI and the US Marshals Service, were looking for them.

Four days before the shooting, Ethan accompanied his father to a local gun shop, where James Crumbley bought a semi-automatic handgun, prosecutors said.

Oakland County Prosecuting Attorney Karen McDonald discussing the latest charges today

Later that day, Ethan posted photos of the gun on social media, writing, "Just got my new beauty today" and adding a heart emoji. His mother posted the next day that the two of them were "testing out his new Christmas present," Ms McDonald said.

Michigan law prohibits those under 18 years of age from buying or possessing firearms, except in limited circumstances such as hunting with a licence and a supervising adult.

"These charges are intended to hold the individuals who contributed to this tragedy accountable and also send a message: that gun owners have a responsibility," Oakland County Prosecutor Karen McDonald told a news conference today.

Prosecutors described several chilling warning signs in the days leading up to the shooting at Oxford High School, about 60 km north of Detroit.

On one day, a teacher saw Ethan Crumbley searching for ammunition on his phone during class and alerted school officials, who left messages for his mother that went unreturned.

In a text message to her son, prosecutors said, Jennifer Crumbley wrote: "LOL, I'm not mad at you. You have to learn not to get caught."

The morning of the shooting, a teacher discovered a drawing that Ethan Crumbley had made depicting a handgun, a bullet, and a bleeding figure.

The words "Blood everywhere" and "The thoughts won't stop - help me" were also written on the sheet, among other messages, according to Ms McDonald.

James and Jennifer Crumbley were summoned to the school, where they were instructed to get Ethan into mental health counseling within 48 hours, she said.

They "resisted" the idea of taking their son home from school and did not search his backpack or ask him about the gun, she said.

"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," she told reporters.