I still go with mob involvement. They had the best mixture of motive, opportunity, and power to get the deed done and not get caught.
In watching the 3 hour program American Experience, they really showed the deterioration and isolation of LBJ in the months leading up to his decisioon not to run. A different man from the one in 1963-87.
BTW watch American Experience in the next two weeks. Oswald's Ghost.
There was a very interesting theory advanced that claimed Johnson declined to run for re-election but was counting on getting the nomination anyway at the convention.