A jury this week awarded $450,000 to a father who has battled conspiracy theorists’ claims that the 2012 shooting death of his son at a Connecticut school was a hoax.

In the years since 6-year-old Noah Pozner was killed, Lenny Pozner has tried to stop people from spreading lies about the mass shooting at Sandy Hook Elementary School and from harassing him and other grieving families for proof that the event happened. In June, Pozner won a defamation case against editors of a book that claimed no one died at the Newtown, Conn., school. A summary judgment found that James H. Fetzer and Mike Palecek defamed Pozner with statements that his son’s death certificate was a fake.

On Tuesday, a Wisconsin jury determined the amount that Fetzer must pay Pozner for making defamatory statements, with the foreperson writing in “$450,000” on the form.

“This is a first,” said Jake Zimmerman, Pozner’s attorney. “This is the first time someone has stood up to these people and gone all the way."

Zimmerman said the case sent an important message that families can stand up for the truth, and it sends a message to those who are spreading the theories.

“Anyone else who’s in the business of harassing families of victims of mass-casualty events — which unfortunately has become a cottage industry, it seems — has to look at this and say there is a cost to making these statements,” Zimmerman said.

Fetzer and others have cast the dispute as an attack on the First Amendment.

Fetzer, a professor emeritus at the University of Minnesota at Duluth, defended the book, which was written by 13 people, and its conclusions.

“This result represents a significant defeat for collaborative research by citizen journalists in an ongoing effort to offset the pervasive influence of ‘fake news’ about a plethora of events, including JFK, 9/11, the Boston bombing and (even) the moon landing,” he wrote in an email Wednesday. “When books are banned, the people are deprived of alternative perspectives in the search for truth, as this case so vividly exemplifies.

“Most Americans, alas, are not in the position to conduct research and sort out truth from fiction, which leaves them vulnerable to narratives that are aggressively promoted by the mainstream media,” Fetzer wrote.

Conspiracy theories began spreading online almost immediately after 26 people were shot and killed at Sandy Hook in 2012, with some speculating the massacre was a staged event to generate urgency for gun-control laws. Even before the funerals were over, grieving families became targets, with people accusing them of being actors paid to play a role. Over time, many assumed they would eventually be left alone, but theories flourished in anonymous online forums and on social media, and Pozner received death threats.

He founded the nonprofit HONR Network to combat harassment, and he and other families sued people who have questioned the attack — including Alex Jones, who runs the conspiracy-theory-driven Infowars website.

Pozner and his attorney went to great lengths at the defamation trial to prove that Noah Pozner was a real little boy who had lived and died: They gave the judge the death certificate, with raised seal, to counter the allegation in the book that it had been faked; turned over scores of pages of pediatric medical records; and submitted DNA samples from Noah and Lenny Pozner.

In September, Pozner reached a settlement with Palecek, according to court documents. Zimmerman said Palecek had agreed to a statement: “The Court has ruled that the death certificate of Noah Pozner is not a fabrication as stated in the book ‘Nobody Died at Sandy Hook.’ I accept the Court’s ruling without appeal and I apologize for any resulting distress that I may have caused.”

In an earlier settlement, the publisher of the book, David R. Gahary, agreed to stop selling it.

Zimmerman said he and Pozner did not suggest a damages amount to the jury for the reputational and emotional harm Pozner suffered. A forensic psychiatrist testified that Pozner had suffered post-traumatic stress disorder after his son’s death, and that the defamatory statements “essentially caused a secondary injury that means his PTSD is chronic and likely will not go away,” Zimmerman said. The jury’s large monetary award “reflects the agony of being harassed in this way,” he said.

Pozner said: “The damages awarded me for Mr. Fetzer’s prolific defamation and harassment are significant, not so much for the dollar amount, which I will likely never see due to Mr. Fetzer’s economic reality, but for the precedent that it sets. This sends a message to hoaxers and conspiracy theorists and others, who seek to use Internet to revictimize and terrorize vulnerable people, that their actions have consequences. When you defame people online, that has consequences.”

The verdict is also important for other victims of online harassment and hoaxes, Pozner said in a written statement, including “victims of other mass casualty incidents, school and church shootings, and high profile murders. Every victory empowers victims of online hate and harassment to stand up and take action, this win gives them . . . tools to do so.”