It’s difficult to prove a negative, but Posner & Bugliosi’s books do a good job of it.
LBJ had a strong motive to kill JFK, but when he begged Earl Warren to investigate he was proclaiming “Whatever you all think, I didn’t do it!”. Warren was both a personal friend of JFK and an incorruptible prosecutor. When his own father was murdered and police couldn’t come up with sufficient evidence, Earl Warren as Alameda County prosecutor ordered the only suspect released, and the case of his father’s murder was never solved.
Well that does it by gosh.
The HSCA was just ig’nant.