Since 1956, the F.B.I. had carried out an expansive campaign to spy on civil rights leaders, political organizers and suspected Communists, and had tried to sow distrust among protest groups. Among the grim litany of revelations was a blackmail letter F.B.I. agents had sent anonymously to the Rev. Dr. Martin Luther King Jr., threatening to expose his extramarital affairs if he did not commit suicide.

“It wasn’t just spying on Americans,” said Loch K. Johnson, a professor of public and international affairs at the University of Georgia who was an aide to Senator Frank Church, Democrat of Idaho. “The intent of Cointelpro was to destroy lives and ruin reputations.”

Senator Church’s investigation in the mid-1970s revealed still more about the extent of decades of F.B.I. abuses, and led to greater congressional oversight of the F.B.I. and other American intelligence agencies. The Church Committee’s final report about the domestic surveillance was blunt. “Too many people have been spied upon by too many government agencies, and too much information has been collected,” it read.

By the time the committee released its report, Hoover was dead and the empire he had built at the F.B.I. was being steadily dismantled. The roughly 200 agents he had assigned to investigate the Media burglary came back empty-handed, and the F.B.I. closed the case on March 11, 1976 — three days after the statute of limitations for burglary charges had expired.