A federal appeals court on Friday upheld the convictions of David Matusiewicz and his sister Amy Gonzalez, who were sentenced to life in prison in the nation's first prosecution of cyberstalking resulting in death.

In a precedential decision, a three-member panel of the U.S. Court of Appeals for the Third Circuit affirmed sentences, as well as the constitutionality of the interstate stalking law that federal prosecutors used to convict the siblings, after their father, Thomas Matusiewicz, killed Matusiewicz's ex-wife and her friend in the lobby the New Castle County Courthouse in 2013.

“As the Court of Appeals observed, this is a watershed case of national importance,” U.S. Attorney David C. Weiss of the District of Delaware said in a statement announcing the verdict. “Cyberstalking is a form of psychological terror that deeply impacts its victims. Individuals who engage in such conduct are on notice that the Department of Justice will prosecute them to the fullest extent of the law.”

Matusiewicz, Gonzalez and their mother were convicted on conspiracy and cyberstalking charges related to the shooting, which also injured two police officers. Thomas Matusiewicz killed himself after exchanging gunfire with police.

The family members were the first people in the United States to be tried and convicted of cyberstalking resulting in death. In 2016, U.S. District Judge Gerald Austin McHugh of the Eastern District of Pennsylvania sentenced all three to life in prison for their roles in what prosecutors said was a three-year campaign to defame and harass Christine Belford before her death.

Lenore Matusiewicz died in May 2016, but Gonzalez and David Matusiewicz appealed their convictions, pushing a range of constitutional and evidentiary challenges.

According to prosecutors, Matusiewicz and Belford were engaged in a bitter custody battle after they divorced in 2006. David Matusiewicz and Lenore Matusiewicz then kidnapped the couple's three girls and absconded to Nicaragua, where the children were found three years later.

The children were returned to Belford, and Matusiewicz and his mother were sentenced to prison after pleading guilty to charges stemming from the kidnapping.

It was from prison, prosecutors said, that Matusiewicz organized a brutal plot targeting Belford. They pointed to a trove of evidence, including emails and other correspondences from the family, accusing her of sexually abusing the children. And they blamed Matusiewicz for setting up a Family Court hearing over parental rights that drew Belford to the courthouse on the day she was killed.

On appeal, Matusiewicz and Gonzalez challenged the cyberstalking statute on First Amendment grounds, arguing that it was too broad. Gonzalez argued that she was expressing a “sincerely held belief” about Belford, and her statements deserved full protection under the First Amendment.

Judge Michael A. Chagares of the U.S. Court of Appeals for the Third Circuit, writing for the court, said there was “overwhelming, uncontradicted evidence” proving that the attacks on Belford had no basis in reality.

“We hold that [the statute] does not violate the First Amendment as applied to Gonzalez, because she did not engage in protected speech. Her conduct was both defamatory and speech integral to criminal conduct,” Chagares wrote.

He was joined in the opinion by Judges Anthony Joseph Scirica and Marjorie O. Rendell.

The 70-page opinion also rejected various challenges to Matusiewicz and Gonzalez's life sentences, including an sentencing enhancement for targeting a vulnerable victim. In his ruling, Chagares said that Belford's three children were especially prone to the effects of the cyberstalking campaign waged against their mother.

“Due to their young age,” he said, “all of these children were more likely to experience substantial emotional distress as a result of the defendants' conduct; they were powerless to protect themselves from allegations of sexual abuse, and as children, were less able to defend and protect themselves against any attempted harm from the adult defendants.”

Matusiewicz and Gonzalez were represented by Edson A. Bostic and Tieffa N. Harper from the Office of the Federal Public Defender.