Answered by Robin Kelley:
In some cases, we know who the informants are(or assassins, whatever the case may be). Actually, in most cases we know--whether we are speaking of Nat Turner and Denmark Vesey or Malcolm X. And the film does name some of the black informants who worked for the FBI and operated within Garvey's establishment. I don't think you can trace a direct line between these informants and the black elite, however. Why they are not always prosecuted to the full extent of the law is linked to who they work for. If they work for "the law," they will escape prosecution, and their ability to infiltrate is precisely why they were hired in the first place.
Some food for thought. Perhaps the more critical "traitors" to the movement in the UNIA were Garvey's officers who pocketed a good deal of the money. Perhaps there is a structural limitation inherent in movements with a hierarchical structure with the goal of accumulation. That individuals could have so much direct access to money is a critical factor in the movement's demise.
Answered by Theodore Kornweibel:
I won't attempt to answer all of this question, but will confine myself to the role of the Bureau of Investigation's black agents and informants in the government's pursuit of Garvey. Their identities can be found in my book, "Seeing Red": Federal Efforts to Suppress Black Militancy, 1919-1925. The black agents were paid the same as white agents, and they joined the Bureau not to "get Garvey," but to secure positions in the federal government which had previously been denied to blacks. They were members of the black middle class who appear to have believed, as did many leaders like W.E.B. Du Bois, Robert Abbott, and A. Philip Randolph, that Garvey was leading blacks toward disillusionment, discouraging them from fighting for civil rights and a future in the United States, and sending a message to white America that blacks were content with segregation.
The agents investigated Garvey (and others), and sometimes acted as agent's provocateurs, because that was their job. The also investigated the black communist and socialist movements, and even the NAACP, because that was their job. The black informants were similar to the black agents, members of the educated "Talented Tenth" who hoped for an integrated future with full civil rights in America, not Africa. They were paid modestly, and sometimes the government balked at reimbursing them for expenses, so they were not in it for the money. Finally, whatever you conclude about the motivations of the black agents and informants, it would be wrong to affix responsibility for their actions to their descendants.
Regarding Medgar Evers, no blacks were involved in his assassination in 1963; it was the work of a Mississippi white supremacist. The reason it took decades to convict his murderer was the unwillingness of white Mississippi juries in the 1960s to convict the killer, Byron de la Beckwith. He was finally convicted of the murder in the 1994, and just recently died in prison.