A little known but extremely important part of the history of domestic surveillance by intelligence agencies is the CACTUS program. CACTUS was a highly classified channel used by agencies like the Central Intelligence Agency and the Federal Bureau of Investigation to transmit information about “the New Left, Black Militants and related matters.” This channel was never disclosed in the Church Committee reports, even when the reports discuss information that was transmitted through CACTUS.

A memo from Howard Zinn’s FBI file shows how the Bureau relied on third-party contractors and press coverage to avoid scrutiny of its surveillance of Vietnam protestors.