In the 1940s, Kennedy helped write the Superman radio show episodes that exposed and ridiculed the Klan's rituals. In the 1950s he wrote I Rode with the Ku Klux Klan, which was later renamed The Klan Unmasked.
Rejected for military service during World War II due to a back injury, Kennedy turned to fight what he called homegrown racial terrorists.
Kennedy acquired membership and trust of the Klan by using the name of a deceased uncle who had been a member. He passed on secrets of the Klan to the outside world including the FBI, Internal Revenue Service, the Anti-Defamation League and Drew Pearson, a columnist for The Washington Post.
He even rummaged through a top Klan leader's wastebasket for evidence that enabled the IRS to press for the collection of an outstanding $685,000 tax lien from the Klan in 1944, according to a New York Times report.
Kennedy testified before a federal grand jury in Miami about the Klan's chain of command in the 1951 bombing death of Harry T. Moore, a Florida leader of the National Association for the Advancement of Colored People, and about bombings aimed at black, Catholic and Jewish centers in Miami, added the report. He also presented evidence in federal court about other Klan bombings and about violence aimed at preventing blacks from voting in the 1944 and 1946 elections.
In 1947, Kennedy provided information - including secret code words and details of Klan rituals - to the writers of the Superman radio program. In 1954, Kennedy wrote his sensational exposé of the workings of the Klan, I Rode With The Ku Klux Klan (later reissued as The Klan Unmasked), which was published by Jean-Paul Sartre.
Kennedy faced accusations, however, of embellishing and sensationalizing his account. His work was featured in the best-selling book Freakonomics by Stephen J. Dubner and Steven D. Levitt. While calling Kennedy the greatest single contributor to the weakening of the Ku Klux Klan, the authors alleged that Kennedy had misrepresented parts of The Klan Unmasked.
Kennedy told the St. Petersburg Times that he did dramatize some of his work to help it reach a wider audience.
It was hardly a cover-up, he told the Associated Press in 2007.
Peggy Bulger, director of the American Folklife Center at the Library of Congress and a friend of Kennedy, vehemently defended him from these accusations.
Exposing their folklore - all their secret handshakes, passwords and how silly they were, dressing up in white sheets, was one of the strongest blows delivered to the Klan, Bulger, who wrote her doctoral thesis was Kennedy's work as a folklorist, commented in a 2007 interview with The Associated Press.
William Stetson Kennedy was born on Oct. 5, 1916, in Jacksonville, Fla., and was related on his mother's side to John B. Stetson, the hat manufacturer. He authored eight books and co-authored a ninth.