The parents of a 15-year-old who shot dead four students at a US high school with a gun bought by his father pleaded not guilty to involuntary manslaughter Saturday, as a Michigan judge set a combined million-dollar bond for their release.

James and Jennifer Crumbley, the parents of shooting suspect Ethan Crumbley, were arrested overnight after police tracked them down in an industrial building in Detroit -- 40 miles (65 kilometers) from the shooting in Oxford.

People embrace as they visit a makeshift memorial outside of Oxford High School in Oxford, Michigan, where a 15-year-old killed four fellow students and wounded seven other people in a shooting Tuesday. The San Juan Unified Schools District warned of shooting threats circulating on Tiktok. In photo: tributes laid out to victims of the Oxford High School shooting. Photo: GETTY IMAGES NORTH AMERICA via AFP / SCOTT OLSON

The pair were considered fugitives -- having reportedly withdrawn $4,000 in cash and switched off their phones -- although the Crumbleys' lawyers told the court their clients "were absolutely going to turn themselves in."

But Sheriff Michael Bouchard of Oakland County -- where the shooting took place -- told reporters during a news conference Saturday that it appeared the couple had help in "hiding" from police.

"They weren't looking toward surrendering at that point," he said.

Oakland County prosecutor Karen McDonald announces that charges have been filed against the parents of the 15-year-old boy arrested for shooting dead four fellow students in Michigan Oakland County prosecutor Karen McDonald announces that charges have been filed against the parents of the 15-year-old boy arrested for shooting dead four fellow students in Michigan Photo: GETTY IMAGES via AFP / SCOTT OLSON

"We believe they were assisted in that location, to get there, to get in," Bouchard said.

"Clearly somebody helped them into that location and made it available to them," even after there was a warrant out for their arrests, he said.

At their arraignment Judge Julie Nicholson set bond at $500,000 for each parent, calling the charges "very, very serious" and saying "the court does have some concern about the flight risk."

These photos released by the Oakland County Sheriff's Office in Michigan show James and Jennifer Crumbley, who face involuntary manslaughter charges after students were shot dead at a high school in a crime in which their son was charged with murder These photos released by the Oakland County Sheriff's Office in Michigan show James and Jennifer Crumbley, who face involuntary manslaughter charges after students were shot dead at a high school in a crime in which their son was charged with murder Photo: Oakland County Sheriff's Office via AFP / Handout

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

The couple's son Ethan has been charged as an adult with state murder and terror charges, while his parents each face four counts of involuntary manslaughter -- each carrying a sentence of up to 15 years in jail.

In announcing charges against the boy's parents on Friday, Oakland County prosecutor Karen McDonald said the rare move was intended to "send the message" that gun owners have a responsibility.

Ethan Crumbley has been charged as an adult with state murder and terror charges after four students were shot dead in Michigan Ethan Crumbley has been charged as an adult with state murder and terror charges after four students were shot dead in Michigan Photo: GETTY IMAGES NORTH AMERICA via AFP / SCOTT OLSON

The Crumbleys are accused not only of supplying their son with a weapon, but of ignoring escalating warnings that he appeared to be on the brink of violence -- including on the day of the shooting.

Four days before the tragedy, James Crumbley bought the 9mm Sig Sauer semi-automatic handgun for his son, McDonald told the judge at Saturday's hearing.

His wife took their son to a shooting range the following day, posting on social media that she had bought a gun for her "baby" for Christmas, the prosecutor said.

Signs in the window of a store show support for the community after the Oxford High School shooting Signs in the window of a store show support for the community after the Oxford High School shooting Photo: AFP / JEFF KOWALSKY

"Instead of disclosing to the school that he had full access to this weapon, they chose not to," she said.

This combination of booking photos obtained from the the Oakland County Sheriff's Office in Michigan on December 4, 2021, shows Jennifer (L) and James Crumbley of Oxford, Michigan This combination of booking photos obtained from the the Oakland County Sheriff's Office in Michigan on December 4, 2021, shows Jennifer (L) and James Crumbley of Oxford, Michigan Photo: Oakland County Sheriff's Office via AFP / Handout

While school shootings carried out by teens have become a sadly familiar part of American life, it is highly unusual for parents to face charges.

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

But the Crumbleys' lawyer Shannon Smith told the judge it was "absolutely not true" that Ethan Crumbley had free access to a gun.

"Our clients are going to fight these charges," Smith said. "Our clients are just as devastated as everyone else."

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 observed Ethan Crumbley searching for ammunition on his cell phone during class.

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

There is evidence the shooting was "premeditated, and he was actually looking forward to it," Bouchard said.

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

The parents were shown the drawing and advised they needed to get the boy into counseling.

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.

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

And James Crumbley called the emergency line 911 to report that a gun was missing from his house and that he believed his son may be the shooter, McDonald said.

According to Bouchard, all three family members are in custody at the Oakland County jail, unable to communicate with each other.