(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, and a 15-year-old suspect is in custody, authorities said. The suspect, who has not been identified by police, is being held at a juvenile detention facility.

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 mass shooting:

How the incident unfolded

Deputies were dispatched to the school at 12:52 p.m. Tuesday, Oakland County Sheriff Michael Bouchard said, noting that more than a hundred 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 9MM Sig Sauer SP2022 semiautomatic pistol, was purchased by the suspect’s father on Friday, four days before the shootings at the school, Bouchard said.

Two 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, who’d earlier said at least a dozen rounds were fired.

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

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

Investigators believe there were no other weapons 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

Investigators identified the victims who died as Madisyn Baldwin, 17, Tate Myre, 16, Hana St. Juliana, 14, and Justin Shilling, 17. Myre died in a patrol car as a deputy was attempting to rush him to the hospital due to the “severity of his wounds,” Bouchard said. Shilling died Wednesday morning, according to the Oakland County Sheriff’s office.

Seven others — six students and a teacher — were shot, Bouchard said. Two were in critical condition, he said Wednesday morning.

Among the wounded were a 14-year-old girl who was on a ventilator following surgery, Bouchard said Tuesday night. A 14-year-old boy also had a gunshot wound to the jaw and head, while the teacher who was shot has been discharged.

There were multiple other non-life-threatening injuries sustained by people as they rushed out of the school, Bouchard said. Most were treated and released at a staging area, he said.

What we know about the suspect

The suspect, a sophomore at the school, is being held at Oakland County Children’s Village — a juvenile detention facility — and is on suicide watch where he is being checked on every 15 minutes, said David Coulter, the Oakland County executive.

“We don’t have a motive at this point in time,” McCabe said Tuesday. “We are still investigating that.”

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.

What we know about the investigation

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

A search warrant was executed at the suspect’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.

“We believe we have some writings that contain some of his thoughts,” Bouchard said Wednesday, adding he didn’t immediately know whether the writings reveal intent.

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.

The suspect could be charged as an adult, a decision that’s up to the Oakland County Prosecutor’s Office, McCabe said.

Prosecutor Karen McDonald office’s has “begun the process of receiving information regarding the investigation” into the shooting, according to a Tuesday statement.

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

The-CNN-Wire

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