Edgar Ray Killen, a former Ku Klux Klan leader who was convicted of the 1964 killings of three civil rights workers in Mississippi, has died in prison at the age of 92, the state corrections department announced.

The one-time Klan leader was serving a 60-year prison sentence for manslaughter when he died on Thursday night in the Mississippi State Penitentiary.

A post-mortem will take place, but no foul play was suspected, the statement said.

His conviction came 41 years to the day after James Chaney, Michael Schwerner and Andrew Goodman, all in their 20s, were ambushed and killed by Klansmen.

The three Freedom Summer workers had been investigating the burning of a black church near Philadelphia, Mississippi.

The killings shocked the nation, helped spur passage of the landmark Civil Rights Act of 1964 and were dramatised in the 1988 movie Mississippi Burning.

The part-time preacher and lumber mill operator was 80 when a Neshoba County jury convicted him of three counts of manslaughter on June 21 2005, despite his assertions that he was innocent.

Killen was the only person to face state murder charges in the case.

He did not say much about the 1964 killings during a 2014 interview with the Associated Press inside the penitentiary.

He said he remained a segregationist who did not believe in racial equality, but contended he harboured no ill-will towards blacks.

Killen said he never had talked about the events that landed him behind bars, and never would.

Long a suspect in the 1964 killings, Killen had made a livelihood from farming, operating his sawmill and preaching to a small congregation at Smyrna Baptist Church in Union, south of Philadelphia, Mississippi.

According to FBI files and court transcripts from a 1967 federal conspiracy trial, Killen did most of the planning in the ambush killings of the civil rights workers.

According to evidence in the 2005 murder trial, Killen served as a kleagle, or organiser, of the Klan in Neshoba County and helped set up a klavern, or local Klan group, in a nearby county.

Nineteen men, including Killen, were indicted on federal charges in the 1967 case. Seven were convicted of violating the victims’ civil rights. None served more than six years.

 Edgar Ray Killen
Killen never spoke about his involvement in the killings (Mississippi Department of Corrections via AP)

Killen’s federal case ended with a hung jury after one juror said she could not convict a preacher.

During his state trial in 2005, witnesses testified that on June 21 1964, Killen went to Meridian to round up carloads of Klansmen to ambush the three victims, telling some of the Klan members to bring plastic or rubber gloves.

Witnesses said Killen then went to a Philadelphia funeral home as an alibi while the attack occurred.

The three bodies were found 44 days later, buried in a red-clay dam in rural Neshoba County.

In February 2010, Killen sued the FBI, claiming the government used a mafia hitman to pistol-whip and intimidate witnesses for information in the case.

The federal lawsuit sought millions of dollars in damages and a declaration that his rights were violated when the FBI allegedly used a gangster known as “the Grim Reaper” during the investigation. The lawsuit was later dismissed.

In the 2014 AP interview, Killen repeated his contention that he was not a criminal, but a political prisoner.

Of one thing he was certain: “I could have beat that thing if I’d had the mental ability.”