The state corrections department announced the death. The cause was not immediately known.
The slayings of the three civil rights workers — James Chaney, Andrew Goodman and Michael Schwerner — were among the most notorious events of the civil rights era and formed the basis for the 1988 film "Mississippi Burning," which starred Gene Hackman and Willem Dafoe as FBI investigators.
The killings occurred in Neshoba County, Miss., which had a long reputation as a center of Klan violence. Mr. Killen, whose family had lived in the area for generations, operated a sawmill, preached at Baptist churches and owned a small farm about 15 miles from the county seat of Philadelphia, Miss.
On June 21, 1964, Chaney, Goodman and Schwerner were in Neshoba County to inspect a black church that had been burned down and to register voters as part of a civil rights effort known as Freedom Summer. As the three men were driving, a deputy sheriff pulled over their station wagon on the pretext of speeding and took them to the county jail. They were released at 10 p.m. and told to get out of the county as fast as possible.
They were followed by two cars filled with Klansmen, who had been alerted and organized by Mr. Killen, according to court evidence. After a high-speed chase on a dark highway, the civil rights workers were overtaken on Rock Cut Road — less than two miles from Mr. Killen's home — forced from their car and shot to death at close range.
Their bodies were not discovered for 44 days. A search led by the FBI eventually found them buried 15 feet deep in an earthen dam on a nearby farm.
Chaney, 21, was an African American from Mississippi. Goodman, 20, and Schwerner, 24, were white New Yorkers. Their deaths, widely chronicled in the media, sparked outrage and were a major turning point in the civil rights movement. Within two weeks, President Lyndon B. Johnson signed the Civil Rights Act of 1964.
Mr. Killen, who was known to be a Klan organizer or "Kleagle," came under immediate suspicion. He was among 18 local men, including police officers, later arrested by federal agents and tried for conspiracy.
Testimony from the 1967 trial described Mr. Killen as the ringleader who coordinated the vigilante Klan group, although he was not present for the killings.
Seven Klansmen were convicted of conspiracy, but Mr. Killen was acquitted. The all-white jury reportedly voted 11-to-1 in favor of convicting him, but the lone holdout said she could "never convict a preacher."
Mr. Killen returned to his normal life in Neshoba County.
He rarely gave interviews, but in 1998 he sat down with the New York Times. Without directly addressing whether he was involved in the killings of Chaney, Goodman and Schwerner, he said: "Those boys were Communists who went to a Communist training school. I'm sorry they got themselves killed. But I can't show remorse for something I didn't do."
Interest in the case was revived by "Mississippi Burning" and by the reporting of Jerry Mitchell of the Jackson (Miss.) Clarion-Ledger. In 1998, Mitchell learned of a secret taped interview in which a Klan leader, Sam Bowers, said "the main instigator" of the 1964 killings was still a free man.
Further investigations led to Mr. Killen, who went on trial for murder in a state court in 2005. Mr. Killen, 80 at the time, was in a wheelchair as he recovered from two broken legs suffered while cutting wood.
Before the jury began deliberating, prosecutors added manslaughter to the original murder charges. In the end, Mr. Killen was convicted of three counts of manslaughter. The verdict was delivered on June 21, 2005 — 41 years to the day after the killings.
Mr. Killen was given the maximum sentence of three consecutive terms of 20 years, for a total of 60 years in prison.
Edgar Ray Killen was born Jan. 17, 1925, in Union, Miss., where his family had been involved in logging, lumber and farming since the 19th century.
Details about his early life are sketchy. He was married twice but had no children. He said he was an ordained minister and was a pastor at several churches.
He was convicted in 1976 of making threats over the telephone to a woman.
Although he did not admit to being a member of the Klan, he had been known to federal investigators and other Klan watchers for decades.
In 1968, after the assassination of the Rev. Martin Luther King Jr. in Memphis, FBI agents knocked on Mr. Killen's door, seeking information. As the agents were about to leave, Mr. Killen asked whether they had identified the killer.
He added, "Man, I just want to shake his hand."
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