Killen, 80, showed no emotion as he was wheeled out of the courtroom in Philadelphia, Mississippi, in a yellow prison jumpsuit to spend the rest of his life behind bars after the judge delivered the maximum sentence. Before announcing his decision, Judge Marcus Gordon said: "You have to remember that I have a job to do, and I have to pass upon a sentence to a person who is 80 years old, a person who has suffered a serious injury."

He said he took no pleasure in delivering what amounted to a life term but added: "The law does not recognise a distinction of age. Should a person 20 years old receive a more severe sentence than a person 80 years old? If so, why?"

The jury, three of whom were black, convicted Killen on Tuesday on three counts of felony manslaughter, finding that he organised a posse to kidnap, beat and shoot Michael Schwerner, Andrew Goodman and James Chaney and bulldoze their bodies under an earthen dam. Their deaths were the basis for the 1988 film Mississippi Burning, starring Gene Hackman.

Mr Gordon sentenced Killen to consecutive 20-year terms on each of the three counts of manslaughter. "I have taken into consideration that there are three lives involved in this case and the three lives should absolutely be respected," he said.

Killen's lawyer said he would appeal, challenging the judge's instruction to the jury to consider manslaughter if they could not agree on murder charges.

Killen's conviction marks the latest in about two dozen "atonement trials" over the past 15 years during which investigators have reopened unsolved cases of murders which took place during the 1960s.

Rita Bender, widow of Michael Schwerner, said the case did not close the book on the crimes of the civil rights era.

"The reality is that what happened today is just the beginning," she said on Court TV.

Killen, who has used a wheelchair since he broke both his legs in a tree-cutting accident in March, will be held in a cell by himself, separated from other prisoners under an administrative protection status reserved for those at risk of retaliation from other inmates. "It's kind of a race issue, in that our [prison] population is 70% black," the Mississippi department of corrections commissioner, Chris Epps, told Reuters.

Killen, who often breathes with the aid of an oxygen tube, will be allowed out of his cell for just one hour a day on weekdays, to shower or exercise, said Mr Epps. He was convicted on the 41st anniversary of the murders.

The three victims had arrived in Neshoba county during the Mississippi Freedom Summer to register African-Americans to vote at the height of the civil rights era. Chaney was a black Mississippian; Schwerner and Goodman were white New Yorkers.

On June 21 they were on their way to investigate the burning of a black church when the local sheriff took them into custody for speeding.

When they were released later that night a posse of Klansmen followed them, murdered them and buried them in a nearby field.

In 1967, 18 men were prosecuted in a federal court on conspiracy charges relating to the case; seven were convicted but none served longer than six years. Among those who walked free from the trial was Killen, the beneficiary of a hung jury. One juror said she could not bring herself to convict a preacher.