Oakland County Undersheriff Michael McCabe said that Justin died around 10am on Wednesday at McLaren Oakland Hospital in Pontiac.
The other three victims in the shooting that occurred on Tuesday have been identified as 17-year-old Madisyn Baldwin, 16-year-old Tate Myre, and 14-year-old Hana St Juliana.
Oakland County Sheriff Michael Bouchard said during a press briefing on Tuesday night that Tate Myre died in a police vehicle on the way to the hospital. He didn’t elaborate on the other deaths.
Sheriff Bouchard said that 11 people were shot by the 15-year-old gunman and that Oxford is a “quiet sweet community that had its innocence shattered”.
Out of the seven victims who are still alive, a 14-year-old boy is in a serious condition after being shot in the head and the jaw. A 17-year-old girl was shot in the neck and is in stable condition. A 15-year-old boy is stable after being hit in the leg. A teacher, 47, was grazed by one of the bullets.
The sheriff added that a 14-year-old girl is on a ventilator after undergoing surgery for gunshot wounds in her chest and neck and that a 17-year-old girl is in critical condition after being hit in the chest.
The shooting is the deadliest on school property in 2021, The New York Times reported.
The suspected shooter, 15, was apprehended by police and a semi-automatic 9mm Sig Sauer handgun was seized.
Mr McCabe said the suspect had not offered any resistance when he was arrested, which he said took place within five minutes of the first 911 call being received, simply putting his hands on his head and surrendering.
A motive for the attack has not yet been established, according to the undersheriff, as the teen invoked his right to an attorney and declined to speak to officers, apparently on the instruction of his parents.
Mr McCabe said investigators planned to examine his smartphone and social media posts as they search for clues pointing to a possible motive and that they have already executed a search warrant on his house, retrieving several items, notably a cache of weapons including long guns.
Speaking to CNN on Wednesday, Sheriff Bouchard said: “It’s clear that he came out with the intent to kill people. He was shooting people at close range, oftentimes towards the head and chest ... It’s just absolutely coldhearted murders.”
Mr Bouchard said investigators were examining writings belonging to the shooter obtained in the middle of the night that contains “some of his thoughts”.
“We can’t get the motive from the suspect that we have in custody, but we think we’ve got a path to get a lot of supportive information as to how and why this occurred,” he said.
Sheriff Bouchard said he was unaware of any previous run-ins with law enforcement by the suspect, adding that investigators had so far seen nothing to suggest a history of disciplinary problems.