What led to the congressional discovery that the FBI had spied on millions of Americans in the 1960s?

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The Church Committee investigations, officially known as the United States Senate Select Committee to Study Governmental Operations with Respect to Intelligence Activities, led to the discovery that the FBI had spied on millions of Americans in the 1960s. This was part of a broader investigation into the activities of the CIA, NSA, and FBI, revealing extensive abuses of power in domestic surveillance.

What Led to the Congressional Discovery That the FBI Had