CNN  — 

School bells were replaced by police sirens Tuesday after a shooting at Michigan’s Oxford High School left four students dead.

Six other students and a teacher were wounded by gunfire. A 15-year-old suspect is in custody and has been charged as an adult, authorities said.

His parents were charged with involuntary manslaughter Friday in connection with their son’s alleged actions, and they were arrested Saturday in Detroit after an hours-long search, authorities said.

The attack was the deadliest US school shooting since eight students and two teachers were slain in May 2018 at Texas’ Santa Fe High School, according to a CNN tally. There have been 48 shootings this year on K-12 campuses, 32 of them since August 1.

Here’s what we know about Tuesday’s shooting:

How the incident unfolded

Deputies were dispatched to the school at 12:52 p.m., Oakland County Sheriff Michael Bouchard said, noting more than 100 911 calls were placed.

As police were swarming the campus, students and staff began barricading doors and hiding in classrooms.

Law enforcement officers quickly entered the building and had the suspect in custody within three minutes of their arrival, the sheriff said.

Once the suspect encountered the officers, he put his hands up and they took a gun from him before placing him in custody.

The weapon deputies said was used in the shooting, a 9 mm Sig Sauer SP2022 semiautomatic pistol, was purchased by the suspect’s father on Friday, four days before the shootings at the school, Bouchard said.

Three 15-round magazines were found at the scene, and the gun had seven rounds of ammunition, Bouchard said. Investigators recovered more than 30 shell casings, said the sheriff.

“We believe he fired at least 30 shots,” he said.

The suspect had 18 rounds left, with seven in his pocket, Bouchard said.

“With this much ammunition still with him … the quick actions of the school and the lockdown as well as the deputies going to the danger, saved lives,” Bouchard said.

An Oakland County sheriff’s deputy assigned to the school helped take the suspect into custody, Undersheriff Michael McCabe said.

Investigators believe no other weapons were involved, and there’s no indication the suspect was wearing body armor.

No shots were fired by responding law enforcement, and the suspect was not injured, McCabe said.

What we know about the victims

The four who died were Madisyn Baldwin, 17, Tate Myre, 16, Hana St. Juliana, 14, and Justin Shilling, 17, officials said. Tate died in a patrol car as a deputy was rushing him to a hospital, Bouchard said. Justin died Wednesday morning, according to the sheriff’s office.

People hug during a vigil Tuesday following a shooting at Oxford High School in Michigan.

Seven others – six students and a teacher – were also shot, Bouchard said.

Among the wounded were a 14-year-old girl who initially was on a ventilator following surgery, but was off the ventilator and in stable condition Wednesday, officials said. A 14-year-old boy also had a gunshot wound to the jaw and head. The teacher, a 15-year-old and 17-year-old boy were discharged, Bouchard said Wednesday.

What we know about the suspect

The suspect is Ethan Crumbley, 15, a sophomore at the school, authorities said. He is being held at the Oakland County Jail.

An ambulance sits at a road block restricting access to Oxford High School following the shooting.

Oakland County Prosecutor Karen D. McDonald said the suspect has been charged with the following:

  • one count of terrorism causing death
  • four counts of first-degree murder
  • seven counts of assault with intent to murder
  • 12 counts of possession of a firearm during the commission of a felony.

Michigan law prevents police from talking to a juvenile without parental permission, and the suspect’s parents have refused that permission and requested a lawyer, Bouchard said.

“It is possible there could be additional charges” when the investigation is complete, McDonald said.

Ethan Crumbley’s parents, James and Jennifer Crumbley, were charged with four counts of involuntary manslaughter and were initially scheduled to be arraigned Friday afternoon.

The Crumbleys were considered fugitives after missing their court hearing. Early Saturday, police said they arrested the couple in a Detroit warehouse after receiving a tip that their car was nearby.

Two attorneys working with the parents contended the couple were not fleeing and intended to turn themselves in Saturday morning. The Crumbleys pleaded not guilty Saturday to the involuntary manslaughter charges, and were being held with bond set at $500,000 each.

Families walk away with from the Meijer's parking lot, where many students gathered after the incident.

What happened before the shooting

The sheriff’s office learned after the shooting two teachers separately reported concerning behavior from Crumbley starting a day before the attack – prompting two meetings with him, including one with his parents just hours before the killings, Bouchard said.

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,” Bouchard said.

“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 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.”

Bouchard declined to detail what the teachers’ concerns were. McDonald told CNN’s Anderson Cooper Thursday there is a “strong possibility” Crumbley had the gun he is accused of using in his backpack during the meeting with school officials and his parents.

“Unfortunately he was allowed to get back to class, and we now know that he had a weapon with him at that time, and that is simply tragic. And it’s my job to hold people accountable who violate the law,” McDonald told Cooper.

When asked why Crumbley was allowed to return to class on the day of the shooting, 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.”

“Prior to those two meetings there was no contact or nothing in his file by either concerning behavior or discipline,” he said.

What we know about the investigation

Investigators have a “tremendous amount of video footage” to review from cameras in the school, Bouchard said.

A parent hugs a child as others come to pick up students following a shooting at Oxford High School.

A search warrant was executed at Crumbley’s home Tuesday, McCabe said. Authorities seized a phone and are examining other seized items, Bouchard said.

Authorities also are investigating pictures of a target and the weapon posted on social media by the suspect, he added.

Investigators found two videos on Crumbley’s cell phone – made the night before the shooting – in which Crumbley talked about shooting and killing students at the high school, sheriff’s Lt. Tim Willis said at the defendant’s arraignment Wednesday.

In addition to the cell phone, a journal was recovered from Crumbley’s backpack detailing his “desire to shoot up the school,” Willis said.

Asked about what Crumbley’s parents might have known about what he was writing or recording before the shooting, or about his accessing the new weapon, Bouchard said: “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.”

Video from the school shows the assailant was “shooting people at close range – oftentimes toward the head or chest,” Bouchard told CNN’s “New Day” on Wednesday. “It’s chilling. It’s absolutely cold-hearted, murderous,” he said.

During the arraignment, prosecutors said video from school surveillance cameras showed Crumbley with a backpack, and 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. It continued for another four or five minutes until he went to another bathroom, Keast said.

The video shows Crumbley “methodically and deliberately” walking the hallways, aiming a gun at students and firing, Keast said.

“What is depicted on that video, honestly judge, I don’t have the words to describe how horrific that was,” Keast said.

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

Correction: An earlier version of this story misspelled the name of Hana St. Juliana.