http://www.prisonplanet.com/articles/january2007/150107ciaman.htm
E Howard Hunt felt Johnson was behind it. See the link to his book before he died.
Now that Lady Bird has passed, truths will start to be revealed. There are many who have written that she was being protected because she was the last of the living.
Interesting idea. There was a story about Hunt in Rolling Stone Magazine (of all places). If I remember it right, I think he confirmed that he is one of the three tramps in the infamous photo. He says the gunman was a Corsican or Frenchman.