Victim: Dr Martin Luther King . The FBI repeatedly deceived Congress by destroying vital files relating to the assassination of Martin Luther King, a new book claims. Leading King historian Stuart Wexler said that the bureau chose to cover up the potential role of a high level informant in the killing rather than tell the truth. Wexler said that it could have been one of the greatest scandals in the history of the FBI - but now we may never know what happened. By destroying the files the FBI disobeyed a direct order not to do so from a Congressional Committee which had been set up to investigate the killing of King and former President John F Kennedy. Wexler told Daily Mail Online that the bureau's actions were 'disturbing' and that his research had left him suspecting that agents had mounted a deliberate operation to bury the truth forever. King was shot dead by James Earl Ray on 4 April 1968 at the Lorraine Motel in Memphis, Tennessee in a killing which galvanized the civil rights movement. Power: Dr King's leadership of the civil rights movement was demonstrated in the I Have A Dream speech in 1963. His death on 4 April 1968 shocked a nation already scarred by the deaths of President John F Kennedy - and the civil leader's murder was followed by that of Robert Kennedy . Guard: The balcony at the Lorraine Motel in Memphis, Tennessee, where Dr King was shot. Caught: James Earl Ray, the assassin of Dr. Martin Luther King Jnr. being taken to his cell by Sheriff William Morris,  in Memphis Tennessee, July 19, 1968. He had been on the run since the death . Key figures: James Earl Ray (left) in the wanted picture issued by the FBI. Joseph Milteer (right) was a racist  known for a covertly recorded interview in 1963 in which he spoke of the Kennedy and King assassinations before they happened . Interest in King's life has been renewed since the release of the Oscar-winning film 'Selma', about the historic march from the Alabama town, which was 50 years ago this year. But Wexler said that there are still many unanswered questions about his death which he addresses in a new e-book called 'Killing King'. The FBI plot began when the House Committee on Assassinations was set up in 1976 - it would finish its inquiries into the killing of King and Kennedy in 1978. Wexler said that the FBI appears to have been spooked after an investigative reporter called Dan Christensen published a number of articles about the Miami connection to both assassinations, articles which the committee were interested in. Wexler said that, according to his research, in 1977 the MURKIN (murder of King) files were destroyed from the FBI field offices in Miami and Mobile, Alabama. Crucially, both had information on Tommy Tarrants, a former high ranking Klan member from Mississippi. Wexler said that something in that file made the FBI upgrade Tarrants from a little-known racist activist to major player in the King assassination to the point where they showed his picture to witnesses at major crime scenes. What that something was, it now seems we will never know. Also destroyed from the Miami field office was the the file on Joseph Milteer, a well-known racist who is known for a covertly recorded interview he gave in 1963 in which he talks about the Kennedy assassination and the King assassination before they happened. Wexler said: 'It's entirely possible that Milteer was raising the bounty money for King's assassination.' The destruction of the files broke a key rule of the FBI's record keeping; that it doesn't get rid of information on people who are still alive. Tarrants was alive then - and he still is today. According to Wexler, every other field office investigation of the King murder was preserved, just as Congress ordered, and eventually transferred to the national Archives and Records Administration. Wexler said: 'I have no doubt this was done deliberately. They are not destroying everybody's files, they are selectively destroying files. 'They wanted Tarrants to give evidence to the committee, they didn't want him to be a suspect. KKK: Tommy Tarrants was a figure in the KKK and seemed initially marginal to the investigation into the MLK murder but was suddenly made a significant figure. Was it because of his links to another more senior figure that files were destroyed? In charge: J Edgar Hoover, the FBI chief, had condemned Dr King and had a mantra that protecting the bureau's reputation was all that mattered. Wexler says his successor stuck to that belief . Dramatic hearing:  James Earl Ray testifies in 1978 before the House Assassinations Committee. At left is Ray's attorney Mark Lane. Ray told the panel he did not kill Martin Luther King, Jr. 'By that point Tarrants had already made a conversion away from radical racist Christianity - he is now a preacher. 'Until then he had been in prison for a bombing and he'd made a full conversion. The FBI arranged for him to get out of prison, which was unheard of.' Wexler said that another reason why Tarrants was problematic is that he led directly to the Klan in Mississippi, which was one of the most violent anti-black chapters of the group at the time. He said: 'Laude Matthews. also known as L.E. Matthews, was due to take over as head of the Klan there and Matthews was a big time deep cover agent for the FBI. 'We can imagine a situation where the FBI does not want the Congressional investigation to lead back to Laude Matthews. 'They did not want to expose him to suspicion. Imagine what it would have looked like if an FBI informant had a connection to the King assassination? 'This is speculation of course, but it would have been was on the worst scandals in the history of the FBI'. If the FBI had covered its tracks over King's assassination, it would fit into the pattern of duplicity and double dealing that marked the bureau's handling of King. The FBI had put him under intense surveillance and bugged his hotel rooms and attempted to destroy his marriage by sending his wife recordings of him with other women. At a press conference in 1964 then FBI director Herbert Hoover went so far as to call King the 'most notorious liar in the country'. Portrayal: David Oyelowo in his acclaimed performance as Dr King in this year's movie Selma . Never forgotten: The memorial at the site of Dr King's assassination.  The anniversary is tomorrow. A few days later one of his deputies, William Sullivan, wrote a vicious letter to King and gave it to an agent in Miami to post to him in Atlanta. It became known as the 'suicide letter' and called him 'filthy' and 'abnormal' and urged him to take his own life. The letter read: 'In view of your low grade, abnormal personal behavior I will not dignify your name with either a Mr. or a Reverend or a Dr. 'And, your last name calls to mind only the type of King such as King Henry the VIII and his countless acts of adultery and immoral conduct lower than that of a beast.' The head of the FBI between 1973 and 1978 was Clarence Kelly, a staunch ally of Hoover. Wexler described Kelly as a man who followed Hoover's mantra that the reputation of the bureau was the most important thing, 'no matter what'. Wexler said that the FBI could still go some way to salvaging its reputation; one way of doing so would be to give King's case to the Justice Department's Civil Rights Cold Case team, which has looked as more than one hundred civil rights crimes, mostly from the 1960s. He also implored the FBI to run the fingerprints it has from the various crime scenes such as rooming house and car used by Ray. The last time it did so was in 2000 when the integrated Automated Fingerprint identification System was in its infancy and could not even recognize palm prints. It also only covered one state and had a tiny database. Wexler said: 'Now the database is three times larger and covers 50 states. They could run the tests in a day. If they want copies of the prints I have them all, digitized.'
FBI accused in new book - by leading King historian Stuart Wexler - of misleading Congress by destroying files on civil rights leader's murder .
He tells Daily Mail Online agency disobeyed direct order to preserve all materials and believes it was to protect informer from being identified .
Tommy Tarrants, high-ranking KKK member went from being little-known racist to having his picture shown to witnesses .
Files from two field offices which had information on Tarrants were destroyed in 1977 after Congress launched Assassinations Committee .
Wexler believes it is because they could have exposed Laude Matthews, head of Mississippi KKK, as an FBI asset under deep cover .