In the late 1960s, as thousands of Americans traveled to Cuba to evaluate the nation’s evolving revolutionary process, the FBI launched a surveillance campaign designed to prove that travel to the communist island by US citizens represented a threat to national security. Focusing on the FBI’s investigation of the Venceremos Brigade, a radical humanitarian organization that sent delegations of Americans to Cuba as volunteers for agricultural and construction projects, this article evaluates the FBI’s claims that Cuba was indoctrinating leftwing Americans with revolutionary theory and training them in guerrilla warfare. But while state surveillance was intended to criminalize the Venceremos Brigade in legal terms and demonize it within the popular imaginary, it failed to reveal any prosecutable evidence of criminality. Instead, the FBI’s efforts inadvertently transformed it into the group’s clandestine biographer, as agents produced a substantial archive of print material on the group. Amassing thousands of pages of surveillance, including rare pamphlets and ephemera, the FBI’s unofficial archive unexpectedly confirmed the liberatory and humanist aspirations of the Brigade. Although there is a dearth of scholarship on the Venceremos Brigade, the longest-lived Cuba solidarity organization in the world, the FBI’s files remain the most extensive archive on the group ever produced, surpassing any university’s holdings. Files on the Venceremos Brigade illustrate the manner in which counter-narratives can surface even within the body of the state’s archives on grassroots political movements, narratives that are potent enough to challenge the power of the state’s evidence deployed against them.