PONTIAC, MICH. -- A prosecutor filed involuntary manslaughter charges Friday against the parents of a teen accused of killing four students at a Michigan high school, saying they failed to intervene on the day of the tragedy despite being confronted with a drawing and chilling message -- "blood everywhere" -- that was found at the boy's desk.

James and Jennifer Crumbley committed "egregious" acts, from buying a gun on Black Friday and making it available to Ethan Crumbley to resisting his removal from school when they were summoned a few hours before the shooting, Oakland County prosecutor Karen McDonald said.

"I expect parents and everyone to have humanity and to step in and stop a potential tragedy," she said. "The conclusion I draw is that there was absolute reason to believe this individual was dangerous and disturbed."

By mid-afternoon, authorities said they were searching for the couple. Sheriff Mike Bouchard said their attorney, Shannon Smith, had agreed to arrange their arrest if charges were filed but hadn't been able to reach them.

Smith, however, said the Crumbleys weren't on the run and had left town earlier in the week "for their own safety."

"They are returning to the area to be arraigned," Smith told The Associated Press.

Earlier, the prosecutor offered the most precise account so far of the events that led to the shooting, three days after four students were killed and others were wounded at Oxford High School, roughly 50 kilometres north of Detroit.

Ethan Crumbley, 15, emerged from a bathroom with a gun, shooting students in the hallway, investigators said. He's charged as an adult with murder, terrorism and other crimes.

Under Michigan law, the involuntary manslaughter charge filed against the parents can be pursued if authorities believe someone contributed to a situation where there was a high chance of harm or death.

Parents in the U.S. are rarely charged in school shootings involving their children, even as most minors get guns from a parent or relative's house, according to experts.

School officials became concerned about the younger Crumbley on Monday, a day before the shooting, when a teacher saw him searching for ammunition on his phone, McDonald said.

Jennifer Crumbley was contacted and subsequently told her son in a text message: "Lol. I'm not mad at you. You have to learn not to get caught," according to the prosecutor.

On Tuesday, a teacher found a note on Ethan's desk and took a photo. It was a drawing of a gun pointing at the words, "The thoughts won't stop. Help me," McDonald said.

There also was a drawing of a bullet, she said, with words above it: "Blood everywhere."

Between the gun and the bullet was a person who appeared to have been shot twice and is bleeding. He also wrote, "My life is useless" and "The world is dead," according to the prosecutor.

The school quickly had a meeting with Ethan and his parents, who were told to get him into counseling within 48 hours, McDonald said.

The Crumbleys failed to ask their son about the gun or check his backpack and "resisted the idea of their son leaving the school at that time," McDonald said.

Instead, the teen returned to class and the shooting subsequently occurred.

"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 -- it's criminal," the prosecutor said.

Jennifer Crumbley texted her son after the shooting, saying, "Ethan, don't do it," McDonald said.

James Crumbley called 911 to say that a gun was missing from their home and that Ethan might be the shooter. The gun had been kept in an unlocked drawer in the parents' bedroom, McDonald said.

Ethan accompanied his father for the gun purchase on Nov. 26 and posted photos of the firearm on social media, saying, "Just got my new beauty today," McDonald said.

Over the long Thanksgiving weekend, Jennifer Crumbley wrote on social media that it is a "mom and son day testing out his new Christmas present," the prosecutor said.

Asked at a news conference if the father could be charged for purchasing the gun for the son, McDonald said that would be the decision of federal authorities.

In a video message to the community Thursday, the head of Oxford Community Schools said the high school looks like a "war zone" and won't be ready for weeks. Superintendent Tim Throne repeatedly complimented students and staff for how they responded to the violence.

He also acknowledged the meeting of Crumbley, the parents and school officials. Throne offered no details but summed it up by saying, "No discipline was warranted."

McDonald was asked about the decision to keep Crumbley in school.

"Of course, he shouldn't have gone back to that classroom. ... I believe that is a universal position. I'm not going to chastise or attack, but yeah," she said.

Asked if school officials may potentially be charged, McDonald said: "The investigation's ongoing."

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Ed White reported from Detroit. The Associated Press journalist Mike Householder in Detroit and David Eggert in Lansing, Mich., also contributed to this report.