A book claims that James Earl Ray shot dead Martin Luther King because of a £62,000 bounty placed on his head by the Ku Klux Klan.

Historians Stuart Wexler and Larry Hancock describe in their new book, The Awful Grace of God, several assassination attempts against King by one of the most dangerous racist group of the time, the White Knights of the Ku Klux Klan of Mississippi.

Using unpublished FBI reports now available thanks to the Freedom of Information Act, the authors revealed that the White Knights put bounties on the prominent activist's life.

Ray, who was arrested at Heathrow airport two months after King's death, confessed that he shot King dead in Memphis in April 1968 but recanted his confession a few days later. He was jailed for 99 years and died in prison in 1998 aged 70.

Experts on the case have always argued that a mix of racism and money pushed Ray to assassinate King. Evidence of a wider conspiracy were immediately dismissed by the FBI, although the select committee on the assassinations of the US House of Representatives found "on the basis of circumstantial evidence" that Ray was likely to be part of a plot.

Wexler told the Daily Beast that it was probably Ray who shot dead King. "He probably did," Wexler said. "But the physical evidence is a morass we didn't really want to get into."

The book's title comes from a famous speech made by Robert F Kennedy after King's murder.