The parents of a gunman who was 17 when he killed eight students and two teachers at his high school in Santa Fe, Texas, in 2018 are not financially liable for his heinous actions, a jury found on Monday.

The verdict, reached after a day of deliberations, followed an emotional three-week trial that was among the first attempts to hold parents accountable in civil court for the actions of their child in a school shooting.

But instead of finding that the parents bore responsibility for the shooting, the jury decided that blame rested with the gunman and with the company that sold him ammunition used in the shooting. The jury awarded hundreds of millions of dollars in damages to the plaintiffs, who included the relatives of several of those killed and others who were wounded.

The trial came several months after a Michigan couple was found guilty of involuntary manslaughter for a mass shooting carried out by their teenage son. In that case, prosecutors presented evidence that the parents had ignored warning signs and failed to lock up a handgun used by their 15-year-old son in an attack at Oxford High School in 2021.

The Texas gunman’s parents, Antonios Pagourtzis and Rose Marie Kosmetatos, were not accused of any crime. The trial instead focused on whether they had been negligent in the storage of more than a dozen firearms in their home — two of which were used in the shooting — and had failed to notice that their son was struggling or take steps to help him.

After the shooting, the gunman, Dimitrios Pagourtzis, was deemed mentally incompetent to stand trial in criminal court, and he remains in a state hospital for mental health treatment. In the absence of a criminal trial, many in Santa Fe, just north of Galveston along the Gulf Coast of Texas, looked to the civil trial as their first opportunity for accountability, six years after the shooting.

“If we’re not going to have gun control laws as it relates to sellers, we at least need laws on how people keep and store their guns if we’re going to protect our children,” said Alton Todd, a lawyer for Rhonda Hart, whose daughter Kimberly Vaughan was killed in the shooting. “These parents stood up and tried, and this jury just didn’t think we met our burden of proof.”

The jury found that the bulk of the responsibility rested with the gunman, with a smaller share belonging to the company that sold him ammunition at a time when he was legally barred from purchasing ammunition. The company, Luckygunner, reached a settlement with the plaintiffs before the trial.

As a result, the company would not pay any portion of the damages awarded by the jury. And the gunman has no ability to pay, said his criminal defense lawyer, Nick Poehl.

“We’re pleased with the jury’s verdict” and with decisions made by the plaintiffs during the trial, he said, particularly “the plaintiffs’ decision to concede the mental illness of Dimitri,” something that they had previously suggested was invented.

Mr. Poehl said that the damages verdict would most likely be the subject of an appeal.

But the trial’s biggest question — and the focus of the plaintiffs’ case — had been whether the parents should be similarly held accountable.

“We’re here because they have refused to accept any responsibility,” Clint McGuire, a lead attorney for the plaintiffs, said in his closing argument on Friday. “It was their son, under their roof, with their guns, who went and committed this mass shooting.”

For nearly three weeks, Mr. Pagourtzis, a Greek immigrant, and his wife, Ms. Kosmetatos, sat in a small county courtroom in Galveston. He remained mostly stone faced; she occasionally dabbed her eyes with a tissue. Both testified.

Mr. Pagourtzis said that the guns had been stored in a glass-and-wood cabinet in the living room and a black metal gun locker in the garage, and that both were kept locked. He said that the keys were well hidden in the house but that he believed his son had managed to find them.

Both he and his wife said that before the shooting they did not notice changes in their son that would have caused them alarm. Ms. Kosmetatos testified that she would have taken action had she known of his troubled mental state.

Relatives of victims and some of the 13 people who were wounded in the shooting watched each day of the proceedings. Some parents wore T-shirts and pins in remembrance of their children. Several were brought to tears on Friday as the judge in the case, Jack Ewing, instructed the jury to consider damages for the victims, and the plaintiffs’ lawyers displayed pictures of the victims.

The trial directly pitted the parents of the victims in the shooting against the parents of the gunman, and those he wounded. One grieving mother brought an urn with her son’s ashes to the courthouse.

“Obviously I’m very disappointed that the parents were not held responsible,” said Flo Rice, one of the plaintiffs, who was working as a substitute teacher on the day of the shooting and was wounded.

The trial, she said, had been “agonizing,” particularly because of the photos shown of the gunman’s face, day after day. “I’ve had nightmares about him every single night since the shooting. He’s coming after me, he’s trying to kill me,” she said. “But now my nightmares are more horrific, I actually see his face.”

As the judge read the verdict, the parents of the gunman wiped tears from their eyes.

“My clients have been quiet and trying to keep their heads low because they knew this community was outraged with them,” Lori Laird, who represented Mr. Pagourtzis and Ms. Kosmetatos, said in an interview after the verdict. “Tony told me he wanted people to know that he’s a good person. He cares about his fellow man.”

The jury was presented with detailed testimony from survivors of their terror during the shooting, and also with descriptions of the seemingly ordinary family life and activities of the gunman, who played clarinet, ran track, and danced with a troupe at his Greek Orthodox Church.

Expert witnesses for the defense testified that the gunman had suffered from schizoaffective disorder, which includes symptoms of schizophrenia, and with manic depression, among other mental illnesses.

One of the witnesses, a psychiatrist who interviewed the gunman after the shooting, said that the gunman had delusions and hallucinations — including that the C.I.A. wanted him to be an assassin — that predated the shooting and continued after.

But, the defense said, he kept his delusions secret.

“Mental illness is an unseen, unknown tidal wave,” Ms. Laird said in her closing argument. “The parents didn’t pull the trigger. The parents didn’t give him a gun,” she said. “Something tragic happened,” she added. “But it’s not because Tony and Rose failed to safely store their guns.”

Mr. McGuire, in his closing, talked about evidence presented from the gunman’s homicidal writings, discovered on his computer by investigators, and pointed to testimony suggesting the parents knew something was wrong with their son but didn’t know how to fix it. He said the parents should have picked up on warning signs: declining hygiene, slipping grades, isolation in his room, wearing a trench coat at school.

Ms. Laird said that the parents were not aware of the writings or that he had been wearing a trench coat, and that the other troubles either were not so severe as to raise concern or at odds with other behaviors.

For example, just days before the shooting, she said, he had performed in Greek dance at a festival. A photo appeared for the jury to see: the gunman in a smiling circle of dancers.