James Earl Ray's campaign was supported by Dexter King, Martin Luther King's son (right)
By pleading guilty in a three-hour hearing in March 1969, he avoided a trial which could have led to a death sentence. Ray then argued that he had been coerced into making the plea and that he had been duped as part of a larger plot.
Prosecutors did not outline a motive for the killing or accuse Ray of being a racist. He repeatedly but unsuccessfully sought the trial that his guilty plea had forestalled.
The missing man
Ray claimed he was set up by a shadowy gun dealer he met in Montreal and knew only as Raoul and said he was off changing a tyre when the shooting happened.
In a report this March, state prosecutors in Memphis said the person identified by Ray as Raoul existed but had nothing to do with the killing. His name was not released.
The US House Select Committee on Assassinations concluded in 1978 that Ray was the killer but that a group of racial bigots in St Louis, reportedly with a $50,000 bounty on King's head, might have been involved, too.
Files closed
The House committee issued a report on the killing but its investigative files are sealed until the year 2029. Civil rights groups have lobbied for those records to be opened.
Ray's last legal effort concentrated on tests he wanted conducted on the rifle that prosecutors say was the murder weapon.
It had been purchased by Ray and was found near the murder scene moments after King was shot, with Ray's fingerprints on it. But Ray claimed it was placed there to frame him.
Ballistics tests by the FBI and a congressional committee in the 1970s failed to prove beyond scientific doubt that the rifle was the murder weapon, although King was killed with a similar gun.
Ray's lawyers argued that more sensitive tests developed since the 1970s might show the gun was not the murder weapon. But tests that were undertaken after a court ruling in 1997 proved to be inconclusive too.
Strange bedfellows
His attempt to get a trial drew an unlikely coalition that included his family as well as King's family and other civil rights leaders who believe King was the victim of a murder conspiracy, not a lone man.
"America will never have the benefit of Mr Ray's trial, which would have produced new revelations about the assassination of Martin Luther King Jr, as well as establish the facts concerning Mr Ray's innocence," Coretta Scott King, Martin Luther King's widow said shortly after Ray's death.
But William Gibbons, the lead state prosecutor in Memphis, said: "About the only thing I can say is I believe the history books will accurately record that James Earl Ray was the killer of Dr King."