Killen, 82, was convicted on June 21, 2005 — exactly 41 years after Michael Schwerner, James Chaney and Andrew Goodman were killed.
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."
Killen was sentenced to serve 20 years for each count, with the three sentences to run consecutively, for a total sentence of 60 years.
The Supreme Court 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.
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.
Killen's attorneys also argued prosecutors knew from a 1967 federal civil rights trial of Killen's involvement but never sought to file state charges. The federal case against Killen ended in a mistrial. However, Killen said the transcript — as well as witnesses — was there for state prosecutors.
Prosecutors said it didn't matter why Killen and others were not prosecuted by the state years ago.
The Supreme Court agreed.
"Killen argues that — because of the low regard for the civil rights of African-Americans held by white juries and politicians in 1964 — he was far less likely to have been convicted in a 1964 trial," Justice Jess H. Dickinson wrote in the court's opinion.
Dickinson said 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.
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 the federal trial.
"Killen, through his counsel, had the opportunity to cross-examine all of the state's witnesses at both trials," Dickinson said.
The Supreme Court's decision in the Killen case was made public the same day as a motions 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 trail date has been set.