It's been a long time, but I read Rush to Judgement when I was a kid. I remember one of the most compelling pieces of evidence for a second shooter in that book were tests conducted by (I believe) an expert Marine marksman that concluded it was physically impossible to shoot and reload the rifle as fast as the shots were fired, not even getting into making accurate shots in the midst of that reloading.
50 years and not 1 deathbed confession, from catholic mobsters? from misguided super-patriots? from former Stasi? Romanians? Give it up.
Yup, his own brother said he did it because he was "just an unloved boy." Also, he had already stalked a general and tried to kill him.