In response to King and other activists, Hoover directed an aggressive FBI operation targeting radical groups that he considered threatening “to the internal security” of the United States. Agents working on Hoover’s operation, called the Counterintelligence Program (COINTELPRO), maintained files on racial justice groups that conducted bombings and assassinations, such as the Black Panthers and the Black Liberation Army. The program also investigated other violent organizations, including left-wing revolutionary groups such as the Weather Underground and the Symbionese Liberation Army, and Puerto Rican nationalist groups such as the Armed Forces of National Liberation (FALN).

But the FBI targeted peaceful activists and activities too. Hoover vehemently condemned the Black Panther’s Free Breakfast for Children program for providing the political group “with a ready audience composed of highly impressionable youths” and called the breakfast program “potentially the greatest threat to efforts by authorities.” Under Hoover, COINTELPRO greatly exceeded the bounds of intelligence-gathering. In 1968, before King’s assassination, the FBI went so far as to send an anonymous blackmail note to King, which appeared to insinuate that he kill himself.