To fix blame for the 1921 riot-massacre, we would have to overlook historians who note that the Klan wasnt even active in Tulsa at the time. Once it was, you might have been hard-pressed to find almost any civic leader who didnt regard it as some sort of Chamber Of Commerce or Fraternal Order.
About that...not believing it. Those 'historians' didn't actually look at history. I was at a place in the late 60's that had things of interest to extremists on both ends of the stupid spectrum. Was 'courted' by the Klan, John Birch Society, American Nazi Party, Communist party, Black Panthers, the JDL - Jewish Defense League, and the Revolutionary Communist Party (splinter group). Just to name a few. FBI was curious about all that attention, too, so I guess you could include them...? Klan at that time was very outgoing about their long running traditions and the continuity down through the ages.