Martin Luther King Jr.’s “I Have a Dream Speech” inspired the world. It also galvanized the Federal Bureau of Investigation into undertaking one of its biggest surveillance operations in history.

Initially approved in October 1963 by then-Attorney General Robert Kennedy, the FBI’s wiretap and hidden-microphone campaign against King lasted until his assassination in April 1968. It was initially justified to probe King’s suspected, unproven links to the Communist Party, morphing into a crusade to “neutralize” and discredit the civil rights leader.

The speech’s impact on the FBI was first outlined in a 1976 report of the U.S. Senate “Select Committee To Study Governmental Operations with Respect to Intelligence Activities,” known by its popular nickname, the “Church Committee,” after Idaho Democrat Frank Church.