tell the co-worker state secrets and tell him not to tell anyone else under penalty of execution for treason... maybe without a trial. then execute some friends of his that did talk. then see if your friend talks.
make it a fair comparison.
Still won’t matter. Human beings talk. That’s what they do. And just who the hell is going to keep their mouth shut for over fifty years. Just the idea of getting people involved at the time would have had people blabbing all over the place. There isn’t a shred of evidence of any ‘’conspiracy’’ other then the one in Oswalds mind.