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Students leave flowers at the sign of Oxford High School where memorial items are being placed in Oxford, Mich., Wednesday, Dec. 1, 2021. A 15-year-old sophomore opened fire at the school, killing several students and wounding multiple other people, including a teacher. (AP Photo/Paul Sancya)

(CNN) — Two teachers separately reported concerning behavior from sophomore Ethan Crumbley starting the day before the deadly Michigan high school shooting he's accused of — prompting two meetings with him, including one with his parents just hours before the killings, a sheriff said Thursday.

Crumbley, 15, was charged as an adult Wednesday with terrorism, murder and other counts in connection with Tuesday's shooting that killed four students and injured seven other people at Oxford High School north of Detroit.

The first behavioral report came Monday, when "a teacher in the classroom where he was a student saw and heard something that she felt was disturbing," Oakland County Sheriff Michael Bouchard told Brianna Keilar on CNN's "New Day."

"And they had a counseling session about it with school officials, and a phone call was left with the parents," he said.

Then on Tuesday — hours before the shooting — "a different teacher in a different classroom saw some behavior that they felt was concerning, and they brought the child down to an office, had a meeting with school officials, called in the parents, and ultimately it was determined that he could go back into class."

Authorities say Crumbley soon opened fire outside a school bathroom. He moved through a hallway at a "methodical pace," prosecutors said, shooting at students in hallways and classrooms before surrendering in what's become the deadliest shooting at a US K-12 school since 2018 and the 32nd on such a campus since August 1.

Bouchard declined to detail what the teachers' concerns were, adding that his department was "never informed of either meeting prior to the shooting or that there were any concerns about behavior."

When asked why Crumbley was allowed to return to class on Tuesday, Bouchard said: "That will all be part of the investigation, in terms of what they thought, and why they thought that that was the right step."

Cell phone videos refer to killing students, officials say

After the shooting, investigators found two videos on Crumbley's cell phone — made the night before the shootings — in which Crumbley talked about shooting and killing students at the high school, sheriff's Lt. Tim Willis said at Crumbley's arraignment Wednesday.

Another piece of evidence investigators uncovered is a journal that was in Crumbley's backpack that detailed his "desire to shoot up the school to include murdering students," Willis said.

The weapon authorities said was used in the shooting, a 9mm Sig Sauer SP2022 pistol, was purchased by Crumbley's father four days before the shooting, Bouchard said Tuesday.

Asked about Crumbley's parents' knowledge of what their son was writing or recording before the shooting or his accessing the new weapon, Bouchard on Thursday told CNN: "We don't have any information that they knew that this was a path he was headed. But ... that's very much an active investigation."

Oakland County Prosecutor Karen McDonald said Thursday that her office is determining whether Crumbley's parents should be charged and she expects an announcement by Friday afternoon.

McDonald has said that firearm ownership comes with legal responsibilities such as securing the gun properly and ensuring ammunition is kept separate.

CNN has pressed the prosecutor to speak on whether evidence was recovered to support potential charges and which charges are being considered. The prosecutor has declined to go into detail, citing the investigation.

CNN has attempted to reach Crumbley's parents and is attempting to identify their attorney.

How authorities say the shooting unfolded

In court Wednesday, prosecutors said video from school surveillance cameras showed Crumbley with a backpack, then a minute later exiting a bathroom without the backpack and with a gun in hand.

Crumbley began firing outside the bathroom, prosecutor Marc Keast said. After students started running away, he proceeded down the hallway at a "methodical pace" and shot inside classrooms and at students who hadn't escaped, Keast said. That continued for another four or five minutes until he went to another bathroom, Keast said.

Video from the school shows the assailant fired at the victims at close range, often aiming at the head or chest, Bouchard told CNN.

When deputies arrived, Crumbley set down the gun and surrendered, authorities have said.

Crumbley was charged with terrorism causing death and four counts of first-degree murder. He also faces seven counts of assault with intent to murder and 12 counts of possession of a firearm during the commission of a felony, McDonald said.

"There's a lot of digital evidence ... there's video, there's things that have been on social media, but you probably don't even need to see that to know how terrifying it is to be in close proximity of another student shooting and killing fellow students. I mean, it's terror," McDonald told CNN of the terrorism charge.

Addressing the first-degree murder charges, McDonald told reporters the accusation "requires premeditation and I am absolutely sure after reviewing the evidence that it isn't even a close call, it was absolutely premeditated."

Crumbley's lawyer entered a not guilty plea on the teen's behalf during the arraignment. The judge scheduled a probable cause hearing for December 13 and a preliminary examination hearing for December 20. He's being held at the Oakland County Jail.

Survivors coping with trauma and grief

As investigators continue to comb through details from the shooting, students who lived through the terrifying ordeal are struggling to cope in the aftermath.

JaVon Pittman, a student at Oxford High, said he took cover as the incident unfolded. "We put the table at the door, barricaded the doors, and we just turned the lights off and we hid under the desk," the 17-year-old told CNN on Wednesday.

He called his father while he was hiding, whispering that there was a shooting at his school, JaVon said. His younger brother, Jonte, was also at the school during the shooting and was able to escape.

"It was devastating to hear a phone call like that," said JaMar Pittman, JaVon's father.

JaVon and his father became emotional recalling the fear they both felt in that moment.

"You try to be there for your kids. You can't be there for your kids, and you get nervous. And for you to be their leader, their father, the superhero, whatever. You can't save your kids. That's devastating," JaMar Pittman said.

JaVon said two of the shooting victims were like brothers to him.

"I was good friends with Justin (Shilling). Me and him used to laugh about everything, we were cool. ... Tate (Myre) was a wonderful kid. He was probably ... the best person that I could probably meet, besides my dad and besides God," JaVon said.

Zander Cumbey, a junior at the high school, told CNN that Myre was his close friend with whom he played football.

"He was a great person, a great leader," he told CNN. "Always wanted to make sure everyone was okay and involved in everything he could."

The victims gunned down at school

The shooting claimed the lives of four teenagers, the youngest being 14.

Hana St. Juliana, 14, was a basketball player at Oxford High.

"We will never forget your kind heart, silly personality, and passion for the game," the team wrote on Twitter. "Since 6th grade camp you have stayed dedicated to Oxford Basketball, soaking in the game."

Justin Shilling died Wednesday morning, the Oakland County Sheriff's Office said. Anita's Kitchen, where the 17-year-old worked, said he was co-captain of the bowling team, a devoted friend and "simply a pleasure to be around."

"Our thoughts are with his family and friends at this difficult time," the statement said.

Madisyn Baldwin, 17, was expected to graduate this year, her family told CNN affiliate WDIV, and had already been accepted to several colleges — some with full scholarships.

"This beautiful, smart, sweet loving girl was tragically taken from us all today leaving a huge hole in all of our hearts ... " Baldwin's grandmother Jennifer Graves Mosqueda wrote on Facebook. "This horrific day could never have been imagined or planned for."

And Tate Myre, 16, was a football player who had been playing for his school's varsity team since he was a freshman, the team said in a tweet. He was also an honor student.

"Tate was a great young man with a bright future and beloved by all. You will be missed, Tate," the team tweeted Tuesday evening.