Killen, 82, was convicted on June 21, 2005 — exactly 41 years after Michael Schwerner, James Chaney and Andrew Goodman were killed. He was sentenced to three consecutive 20-year prison terms.
The three victims, all young men, had been helping blacks to register to vote in Neshoba County when they were killed. Their bodies were found two months later buried in an earthen dam.
Witnesses testified that Killen helped plan the slayings. The case was portrayed in the 1988 movie "Mississippi Burning."
In his appeal, Killen argued that in the 1960s, he wouldn't have been convicted by a jury of his peers of any crime under the evidence presented in 2005. He said prosecutors waited for an opportune political climate in which to indict.
Justice Jess H. Dickinson wrote in the court's opinion the political climate may have changed but Killen failed to show how this prevented him from defending himself.
"We find this argument has no merit, and we are surprised it is made," Dickinson said.
The high court also rejected Killen's arguments that he had nothing to do with the slayings. The court said testimony throughout his 2005 trial showed Killen directed the Klan attack on the three men and gloated about it afterward.
On the issue of waiting four decades to bring Killen to trial, Dickinson said there is no statute of limitations on the crime of manslaughter in Mississippi. Dickinson said the delay was not prejudicial to Killen.
Dickinson said of the prosecution's 14 witnesses, eight testified in person at the 2005 trial and the testimony of six others was taken from the transcript of a 1967 federal trial that had ended in a mistrial.
"Killen, through his counsel, had the opportunity to cross-examine all of the state's witnesses at both trials," Dickinson said.
Also on Thursday, a hearing was under way in another civil rights case dating back to 1964.
U.S. District Judge Henry T. Wingate on Thursday denied a request by attorneys for reputed Ku Klux Klansman James Ford Seale to prohibit the use of a questionnaire designed to reveal the racial attitudes of potential jurors for Seale's upcoming trial.
Seale, 71, is charged with kidnapping in the 1964 slayings of two black teenagers in southwest Mississippi. No trial date has been set.