The new investigation rejected allegations that arose in recent years from former Memphis bar owner Loyd Jowers, former FBI agent Donald Wilson and earlier from Ray himself that a mysterious man named Raoul or others, including federal agents, police or black ministers, participated in a plot to kill King in 1968.
In a 138-page report government investigators concluded they'd found "no reliable evidence that Dr. King was killed by conspirators."
"Nor have any of the conspiracy theories advanced in the last 30 years, including the Jowers and the Wilson allegations, survived critical examination," the report read.
Four earlier investigations reached similar conclusions.
The King family disputed the findings. Martin Luther King III said, "James Earl Ray was not the triggerman and, in fact, was an unknown patsy."
Ray, who pleaded guilty to the murder in 1969 and died in prison two years ago, claimed he was merely the fall guy in a broader scheme. In a 1997 jailhouse meeting, King's son, Dexter, seemed to accept Ray's argument.
Ray did not make the claim alone. In 1993, Jowers, who owned a tavern across the street from the motel room where King was shot, said a produce dealer involved with the Mafia gave him $100,000 to hire an assassin and assured him Memphis police would not be around.
Jowers, who died last month, claimed someone whose name sounded like Raoul gave him a gun and the assassin fired from behind Jowers' bar, not from a rooming house window above it where Ray had stayed.
Justice investigators said Jowers told this story only once under oath and later repudiated that version, and contradicted himself in many retellings.
Wilson, the former FBI agent, claimed he found papers in Ray's car referring to a "Raoul".
The Justice investigators said "Wilson had given materially inconsistent accounts." He later claimed to have found but did not produce three other documents, including one with the FBI's Atlanta telephone number.
Wilson "gave contradictory stories about whether and which documents were allegedly later stolen from him," the report said.
The investigation, headed by one of the department's leading civil rights prosecutors, Barry Kowalski, concluded, "We found nothing to disturb the 1969 judicial determination that James Earl Ray murdered Dr. King or to confirm that Raoul or anyone else implicated by Jowers or suggested by the Wilson papers participated in the assassination."
Prodded in part by the King family's own embrace of some of these theories, Attorney General Janet Reno ordered the new prbe Aug. 26, 1998, even though the assassination had previously been studied by two Justice Department investigations, a U.S. House committee and the Shelby County, Tenn., district attorney's office.
Last December, a civil court jury in Memphis ruled in favor of the King family, which had sued Jowers for wrongful death. That jury concluded that Jowers and "others, including government agencies" conspired to assassinate King.
King's son Dexter said after that verdict: "We know what happened. This is the period at the end of the sentence."
But the Justice Department report said the Memphis civil trial last year "featured a substantial amount of hearsay evidence purporting to support the existence of various far-reaching, government-directed conspiracies to kill Dr. King."
The Justice investigators also found that, "There is no reliable evidence to support the allegations presented in King vs. Jowers of a government-directed conspiracy involving the Mafia and Dr. King's associates."
The report said there wasn't enough evidence to warrant investigating suggestions by the House Select Committee on Assassinations in 1979 and the Shelby County district attorney in 1998 that Ray's surviving brothers may have conspired with him.
While the Justice Department Friday dismissed all of the conspiracy claims surrounding the King murder and recommended no further investigations "unless and until reliable substantiating facts are presented," the report also recognized that doubts will linger.
"Questions and speculation," it concluded, "may always surround the assassination."
CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report