By Jay Croft, CNN
(CNN) -- The parents of the 15-year-old charged in this week's deadly shooting at Michigan's Oxford High School were each charged Friday with four counts of homicide involuntary manslaughter.
"It's been a devastating week for us," Oakland County District Attorney Karen McDonald said at a press conference. "There are other individuals who contributed ... and it's my intention to hold them accountable as well."
In this rare instance of charging parents, prosecutors will try to prove that James and Jennifer Crumbley were criminally negligent and contributed to a dangerous situation that resulted in the deaths of four teenagers Tuesday.
McDonald said James Crumbley bought the gun four days before it was used in the shooting. His son, sophomore Ethan Crumbley, 15, was with him and later posted on social media about the gun, calling it "my new beauty."
Jennifer Crumbley also posted about the gun on social media, calling it "his new Christmas present," McDonald said.
Those are among the chain of events that led up to the shooting, McDonald said.
Oakland County Sheriff Michael Bouchard said the boy could not legally own the gun or carry it to most places -- with rare exceptions such as shooting ranges.
Ethan Crumbley has been charged as an adult with terrorism, murder and other counts in the rampage north of Detroit that also left seven people wounded. The shooting was the deadliest at a US K-12 campus since 2018 and the 32nd such attack since August 1.
His parents are expected to be arraigned at 4 p.m. Friday on four counts of involuntary manslaughter.
Lt. Tim Willis, head of the special investigations unit at the Oakland County Sheriff's Office, said that officers were planning to arrest the parents.
Addressing reporters, McDonald said she was charging the parents in part to make a point about the responsibility of gun ownership, but that the facts of the case were egregious on their own.
"I'm angry as a mother. I'm angry as the prosecutor. I'm angry as a person that lives in this county," she said. "There were a lot of things that could have been so simple to prevent and, yes, there was a perfectly executed response, and he was apprehended immediately and we have great law enforcement and good training.
"But as I said before, four kids were murdered, and then seven more injured, so, yes, I think we should all be very angry."
Prosecutor outlines a chilling progression of events
McDonald said a teacher saw Ethan Crumbley searching ammunition on his cell phone during class the day before the shooting and reported it to school officials. The school contacted Jennifer Crumbley via voicemail. Officials also sent an email but received no response from either parent, McDonald said.
"Jennifer Crumbley exchanged text messages about the incident with her son on that day, stating, quote, 'LOL, I'm not mad at you. You have to learn not to get caught.' End quote," McDonald said.
The day of the shooting, a teacher found a note on Ethan Crumbley's desk that alarmed her so much she took a picture of it on her phone, McDonald said. The note included a drawing of a semiautomatic handgun pointing at the words, "The thoughts won't stop. Help me," she said.
Another section was a drawing of a bullet with the words "Blood Everywhere" written above it. Between the drawing of the gun and the bullet is one of a person who seems to have been shot twice and is bleeding, McDonald said.
"Below that figure is a drawing of a laughing emoji," McDonald said.
Also found on the note, according to McDonald: "My life is useless" and "The world is dead."
James and Jennifer Crumbley were "immediately summoned to the school," McDonald said. A counselor met with the parents and the boy, who had altered the drawing by scratching out the drawings of the gun and bloody figure, along with the words, according to McDonald.
The parents were told to get their son into counseling within 48 hours, McDonald said. Neither parent asked their son to see the gun or "inspect his backpack for the presence of the gun which he had with him," according to McDonald.
The parents left school and the boy was sent back to class.
When news broke about the shooting, according to McDonald, Jennifer Crumbley sent her son a text: "Ethan, don't do it." James Crumbley called 911 to report a gun was missing and his son might be the shooter.
McDonald said investigators found that the gun used in the shooting had been stored unlocked in a drawer in the couple's bedroom.
School officials: No disciplinary action was needed.
When the meeting was held before the shooting, no disciplinary action was warranted, the school district superintendent said Thursday.
"No discipline was warranted. There are no discipline records at the high school," Tim Throne, the superintendent of the Oxford Community Schools, said Thursday in a video statement. "Yes, this student did have contact with our front office. And yes, his parents were on campus November 30th."
There is a "strong possibility" Crumbley had the gun he allegedly used in the shooting in his backpack during the meeting, McDonald told CNN Thursday night.
The school's video surveillance cameras will let investigators "really map out exactly and literally watch what the perpetrator did" from that meeting through the shooting and being taken into custody, Bouchard, the Oakland County sheriff, told CNN on Friday morning.
The shooter had the gun "on his person or in his backpack or somehow secreted" it away from a school location, he said.
Another warning sign had come Monday -- the day before the shooting -- when a different teacher "saw and heard something that she felt was disturbing" related to Crumbley's conduct in the classroom, Bouchard said. School officials held a counseling session with Crumbley about the behavior in question, and his parents were notified by phone, Bouchard added.
Crumbley's defense attorney asked the court Wednesday to enter a not guilty plea on his client's behalf. CNN has attempted to reach Crumbley's parents and is trying to identify their attorney and a new attorney for their son.
School is like a 'war zone,' superintendent says
Two days after the deadly attack, Oxford High School is "like a war zone," Throne told his community from the school in the 13-minute video posted on YouTube.
"This high school is a wreck right now," he said, adding repairs could take weeks.
The shooting claimed the lives of Madisyn Baldwin, 17; Tate Myre, 16; Hana St. Juliana, 14; and Justin Shilling, 17, officials have said.
Throne commended students and staff for how they handled an active shooter threat, during which some administrators performed CPR and students used desks and chairs to barricade themselves inside classrooms for protection.
More than 100 calls to 911 were made to report the shooting as police rushed to the school at 12:52 p.m. local time, Bouchard said. Within "two to three minutes" of officers' arrival, the shooter had surrendered.
"I believe they literally saved lives, having taken down the suspect with a loaded firearm still in the building," the sheriff said.