In our ongoing series, “If Lee Harvey Oswald Didn’t Kill JFK, Then Who Did?”, we’ve examined some of the men who have been accused over the years of taking part in the assassination of President John F. Kennedy. This week, we turn our attention to one of the more intriguing—and controversial—names connected to the case: Herminio Díaz García.

Díaz García was a Cuban exile, anti-Castro militant, and a man reportedly experienced with weapons and political violence. Years after the assassination, allegations emerged that placed him not simply somewhere on the fringes of the JFK story—but in Dallas on November 22, 1963, as a participant in the assassination itself.

But where did that allegation come from? Who claimed Díaz García was involved? What evidence supports the story, and what evid ...