There have been a number of books written which tie Hoover and LBJ to Kennedy's murder. Most notably: BLOOD, MONEY, HONOR AND POLITICS: HOW LBJ MURDERED JFK. By Barr McClellan.
Don't look for it on too many bookshelves. The Johnson family stopped its publicity. McClellan was a partner of Ed Clark, LBJ's personal attorney. Clark told Mclellan (father of Bush pr sec before Snow), that Clark set it up. Johnson, HL Hunt, Hoover, some in the Secret Service, were all involved. It was about money and Johnson facing prison over the Bobby Baker scandal.
LBJ was off the ticket in '64 and headed for prison.
McClellan was on cspan and said his pre publicantion appearances on the networks were all cancelled because the family stopped them.
Thanks for the info! I will try to hunt up the book!