DOVER, Del. (AP) — A federal court on Friday rejected the appeals of a Delaware man and his sister who were sentenced to life in prison in a landmark cyberstalking case involving the killing of the man's ex-wife.

In a 77-page ruling, a three-judge panel of the Third U.S. Circuit Court of Appeals in Philadelphia upheld the convictions of David Matusiewicz, 51, and his sister, Amy Gonzalez, 46, for what prosecutors say was a three-year campaign of torment and stalking against Christine Belford.

Belford, and a friend, Laura "Beth" Mulford were fatally shot by the siblings' father, Thomas Matusiewicz, at the New Castle County Courthouse in February 2013. Mulford had accompanied Belford there for a child support hearing involving the three daughters Belford had with David Matusiewicz.

Thomas Matusiewicz, 68, then exchanged gunfire with security officers before killing himself.

Prosecutors say Belford's shooting capped a long, orchestrated effort by the Matusiewicz family to harass and intimidate her in an effort to regain custody of the children.

David Matusiewicz and Gonzalez were convicted, along with their mother, Lenore Matusiewicz, on federal conspiracy and cyberstalking charges. Lenore Matusiewicz died in prison in 2016. The case marked the first convictions in the nation for the charge of cyberstalking resulting in death.

Prosecutors claimed that David Matusiewicz, a former optometrist, conspired with his parents and sister over several years to torment and stalk his ex-wife with the intent to injure, harass, intimidate and kill her.

Thomas Matusiewicz's family members consistently denied knowing that he intended to kill Belford.

Attorneys for Gonzalez and David Matusiewicz argued on appeal that some of the evidence used by the prosecution was protected speech under the First Amendment, and that jurors shouldn't have heard certain details contained in a family court order terminating David Matusiewicz's parental rights. They also claimed the trial judge made several errors regarding jury instructions and the admission of evidence.

The appeals court panel rejected all their challenges, lauding U.S. District Judge Gerald McHugh Jr. for his "outstanding work" in handling a complicated case "with exceptional precision and care."

"As the Court of Appeals observed, this is a watershed case of national importance," U.S. Attorney David Weiss said in a prepared statement. "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."

Edson Bostic, a federal public defender representing David Matusiewicz, declined to comment on the ruling. Jeremy H.G. Ibrahim, an attorney representing Gonzalez, did not immediately respond to an email seeking comment.

David Matusiewicz had his parental rights stripped in 2011 after he pleaded guilty in 2009 to federal fraud and kidnapping charges after he and his mother took his daughters to Central America. Lenore Matusiewicz served more than a year in state prison for her role in the 2007 kidnapping. The Matusiewicz family has said they were trying to protect one of the daughters from being sexually abused by Belford, an assertion that has never been proved.

The defendants nevertheless continued to maliciously accuse Belford of abuse and neglect in a series of online postings, emails and letters, according to prosecutors.

During the cyberstalking investigation, authorities also found a purported "hit list" that included the names of judges and attorneys who were involved in the family court disputes between the former spouses, as well as the name of the federal judge who sent David Matusiewicz to prison on kidnapping and bank fraud charges.