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, formally known as the United States Senate Select Committee to Study Governmental Operations with Respect to Intelligence Activities, was formed in 1975 to investigate the intelligence gathering activities of the CIA, NSA, and FBI. The investigation uncovered evidence that the FBI had engaged in widespread and illegal spying on American citizens during the 1960s and 1970s, including on civil rights activists, anti-war protesters, and political dissidents. The findings of the Church Committee led to reforms in intelligence gathering practices and increased scrutiny of government surveillance activities.

What Led to the Congressional Discovery That the FBI Had