“My face-to-face interactions with Mr. Pozner have led me to believe that Mr. Pozner is telling the truth about the death of his son,” Mr. Gahary said. “I extend my most heartfelt and sincere apology to the Pozner family.”

The ongoing battle by the families of Sandy Hook victims to stop disinformation about the attack shows how persistent conspiracy theories about the massacre remain nearly seven years after one of the deadliest mass shootings in American history.

Mr. Pozner is also suing Alex Jones, the right-wing conspiracy theorist and founder of Infowars, over his assertions that the massacre was an elaborately staged ruse meant to promote gun control. That case is proceeding in Texas. Separately, another case against Mr. Jones brought by relatives of five children and three adults killed in the shooting, along with one F.B.I. agent who responded to the scene, is proceeding in Connecticut.

In 2016, a Florida woman, Lucy Richards, was sentenced to five months in prison for sending Mr. Pozner death threats. She was also banned from visiting websites run by conspiracy theorists, including Mr. Fetzer’s.

On Monday, lawyers said in court documents in the Connecticut case that Mr. Jones’s legal team had included an image of child pornography as part of his legal filings in that case. Mr. Jones denied the allegation, calling it an attempt to frame him.

The legal challenge in Wisconsin focused on Mr. Fetzer, a retired professor at the University of Minnesota Duluth, who lives in Dane County, Wis., and has been asserting for years that the Sandy Hook killings were staged. Along with his co-editor, Mr. Fetzer has also advanced conspiracy theories about the Sept. 11 attacks, John F. Kennedy’s assassination and other events. He is alleged to have zeroed in on Mr. Pozner as being involved in the conspiracy, leading Mr. Pozner to experience harassment and threats.

To prove the case, Mr. Pozner’s legal team provided a certified copy of Noah’s death certificate, as well as DNA evidence from the Connecticut medical examiner showing that he was Noah’s father, said his lawyer, Jake Zimmerman.