If the teenager who killed four students at his Michigan high school and pleaded guilty to first-degree murder had been a few years older, there would be no question: He would be going to prison for the rest of his life.

But because he was 15 when he opened fire in 2021 in the hallways of Oxford High School, his fate is less certain.

In a hearing that started on Thursday, a state judge heard arguments on whether the gunman, Ethan Crumbley, should be eligible for a sentence that could allow him to one day leave prison. Though most adults convicted of first-degree murder in Michigan must be sentenced to life in prison without parole, the most serious penalty available under state law, a U.S. Supreme Court ruling from 2012 requires judges to consider giving juvenile defendants a chance for eventual release.

The hearing, which is expected to stretch into Friday and possibly next week, could provide the fullest airing yet of the evidence in a mass killing that renewed long-festering questions about gun control, school safety and parental responsibility. And Thursday’s session offered a preview of prosecutors’ separate cases against the gunman’s parents, who are charged with involuntary manslaughter and accused of missing chances to intervene before the shooting. The parents have pleaded not guilty and are awaiting trial.

The proceeding this week, known as a Miller hearing, will require Judge Kwamé Rowe of the Oakland County Circuit Court to consider a number of factors, including the role that the defendant’s age, family and home environment might have played in the crime, as well as the possibility that he might eventually be rehabilitated.

Prosecutors have argued that he should never be eligible for release despite the limits imposed by the Supreme Court in the 2012 case, Miller v. Alabama.

“Almost all of us have come to believe that a juvenile sentenced to life without parole should be exceedingly rare — and I believe that, I embrace that,” Karen D. McDonald, the chief prosecutor in Oakland County, said in court on Thursday.

But she added that “this is an offense unlike any other in this country or state,” and that it was the rare juvenile case in which appellate courts would consider a life-without-parole sentence appropriate.

The defendant, now 17, pleaded guilty last year to murder, attempted murder and terrorism in the shooting at his school, about 45 miles north of downtown Detroit. He killed four students — Madisyn Baldwin, 17; Tate Myre, 16; Justin Shilling, 17; and Hana St. Juliana, 14 — and injured several others. The gunman said in court last year that he had asked his father to buy the 9-millimeter handgun used in the killings, and that it had been left unlocked in the home.

The gunman’s lawyers sought, without much success, to limit evidence and testimony at this week’s hearing. They have also described their client as a troubled teenager whose difficult upbringing makes him exactly the sort of young person who should have a chance to leave prison.

Paulette Michel Loftin, a lawyer for the gunman, said that untreated mental illness and negligent parenting helped contextualize, though not excuse, her client’s actions. She asked the judge to spare him a life sentence, arguing that “it will be clear to you that 15-year-old Ethan Crumbley is not one of those rare juveniles that is irreparably corrupt and without the ability to be rehabilitated.”

The Supreme Court’s decision 11 years ago came as part of a broader rethinking of how juvenile defendants are prosecuted and punished, especially as research has shown significant differences in the development and rehabilitation potential of juvenile criminals compared with adults. The court had previously banned the death penalty for crimes committed by minors.

“When it comes to juveniles, there really isn’t a good match between how serious the actual incident offense is and the likelihood that they’re going to reoffend,” said Dr. Jennifer Piel, a psychiatrist and the director of the Center for Mental Health, Policy and the Law at the University of Washington. “And I think that the Supreme Court in Miller had some appreciation of that.”

In the years since the Miller decision, some states have removed life in prison without parole as a possible punishment for minors. Others, like Michigan, have kept it as an option.

The hearing on Thursday included graphic descriptions of violence and extensive readings from the gunman’s journals, in which he described struggles with mental illness and detailed his plans to kill others. In those journals, a sheriff’s lieutenant testified, the gunman contemplated being sentenced to life in prison and expressed a desire to become famous for his crimes.

Judge Rowe is not expected to rule immediately on whether the teenager can be sentenced to life without parole. And whatever he decides on that question, there will be a separate sentencing hearing, during which victims will have a chance to speak.

Outside the courtroom, the killings at Oxford High have remained a source of pain and anger across Michigan. When state legislators passed several new gun restrictions this year, including one law setting additional requirements for gun storage, they referred to the 2021 shooting.