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 .