The trial of Jennifer Crumbley, whose son carried out the worst school shooting in Michigan history, began on Thursday with dueling portraits: of a negligent mother whose indifference caused a tragedy, and of a good, even “hypervigilant” mother who was in the dark about her son’s troubles until after that tragedy unfolded.

Lawyers for the prosecution and defense sketched out those starkly different images before jurors in a courtroom in Pontiac, Mich., about 20 miles south of Oxford High School, where the mass shooting took place on Nov. 30, 2021.

The shooter, Ethan Crumbley, who was 15 at the time, killed four students and injured seven others. He pleaded guilty to 24 charges, including first-degree murder, and was sentenced last month to life in prison without the possibility of parole.

The case against Ms. Crumbley and her husband is at the leading edge of a push by some prosecutors to hold parents accountable when they are suspected of enabling deadly violence by their children. Just in the last few months, parents whose children carried out gun violence in other states have pleaded guilty to charges of reckless conduct or neglect.

In the Michigan trial, Ms. Crumbley, 45, faces more serious charges: four counts of involuntary manslaughter. Prosecutors say that despite glaring signals of Ethan’s violent intentions, his mother’s failures to take “just ordinary care” to act on what she knew made her criminally liable for the carnage at Oxford High School.

Her husband, James Crumbley, 47, has also been charged and will be tried separately in March. Unable to post a combined $1 million bail, both parents are being held in the Oakland County Jail.

In his opening statement on Thursday, Marc Keast, an Oakland County prosecutor, emphasized that Ms. Crumbley was not charged with murder, nor was she simply accused of being a bad parent. But, he said, because of her “willful disregard for the danger that she knew,” she was a cause of the mass shooting that Ethan carried out.

“Jennifer Crumbley didn’t pull the trigger that day,” Mr. Keast said. “But she is responsible for those deaths.”

Mr. Keast spent much of his opening statement describing the litany of troubling signals leading up the shooting — information that he said the Crumbleys knew but did not share with the school — while suggesting that prosecutors would also highlight Ms. Crumbley’s actions in the aftermath. After Ethan’s arrest, the Crumbleys fled the area, and the police later found them in the basement of a Detroit art studio.

“Her first instinct was to lie,” Mr. Keast said. “Her second was to run.”

Shannon Smith, a lawyer representing Ms. Crumbley, said the prosecution was trying to lay blame for a heinous attack on a woman who “did the best she could” in raising her son.

Ms. Smith said Ms. Crumbley learned only after the shooting about many of the alarming signs, such as Ethan’s text exchanges with friends, that the prosecution would point out as evidence of Ethan’s mental descent. Ms. Smith said that Ethan hid these signs from her and that school officials had never told her about some of the more troubling instances of Ethan’s behavior.

“He did something she could have never anticipated or fathomed or predicted,” Ms. Smith said.

A long record of motions, rulings and other filings leading up to the trial portrays a chaotic home in which, prosecutors say, Ethan’s mental breakdown was ignored and his pleas for help went unheeded.

The week before the shooting, Mr. Crumbley took Ethan to buy a 9 millimeter SIG Sauer handgun, and Ms. Crumbley took him for target practice. On the morning of Nov. 30, both parents were called to the school because Ethan had drawn violent images of a shooting on some class work. Against the suggestion of a school counselor, they did not take their son out of school to get immediate medical help, and they were unaware that he had taken the gun to school in his backpack that day.

Within hours of that meeting, Ethan began shooting.

Ms. Smith said that Mr. Crumbley was responsible for storing the gun. She also said that her client never even considered Ethan a risk for committing violence against others, even after she was alerted that a mass shooting had taken place at the high school. Until authorities explicitly told her what had happened, Ms. Smith said, “it still has not crossed her mind that he would ever shoot another person.”

She also insisted that the Crumbleys went to Detroit after the shooting because they were facing death threats and that they had planned to turn themselves in after learning they were facing charges.

Ms. Crumbley’s lawyers had planned to call Ethan to testify at the trial, along with three of doctors who had spoken with him. But lawyers representing Ethan told the court in a letter that they advised him to “invoke his right to remain silent,” and to assert protections when it comes to confidential information, including conversations with treatment providers.

Ms. Crumbley’s lawyers responded by asking the judge to compel Ethan and the doctors to testify. The judge hasn’t ruled on the question.

After the opening statements on Thursday, Molly Darnell, a teacher at Oxford at the time of the shooting, took the stand as the first witness. She described that day: the commotion, the loud pops, the announcement of a lockdown and the terrifying moment when she went to close her door and locked eyes with Ethan. He shot her in the arm.

Ms. Darnell, recalling that moment, broke down in tears. “I couldn’t wrap my head around what was happening,” she said.