… The Church Committee, along with decades’ worth of Freedom of Information Act releases, exposed once top-secret documents that FBI executives never imagined would see the light of day. These files detailed the scale of politically motivated misbehavior that had occurred when executive-branch controls and meaningful congressional oversight were absent. As a historian who cut his teeth on that copious record, I found it unimaginable that congressional Democrats, or American progressives generally, would ever return to championing unquestioned acceptance of FBI claims that its surveillance practices must remain hidden from the public.

But as Frank Church’s legacy faded, the FBI protested that the 21st-century bureau bore no relationship whatsoever to Hoover’s. In a 2016 speech, then-Director James Comey said that under the glass on his desk he kept a copy of a 1963 memo, signed by Attorney General Robert F. Kennedy, authorizing a wiretap on Martin Luther King Jr. “I keep it there in that spot,” Mr. Comey said, “to remind me of what we in the FBI are responsible for, and what we as humans are capable of, and why it is vital that power be overseen, be constrained, be checked.”

Yet anyone eager to embrace the belief that today’s FBI is a rigorously professional and politically unbiased agency is overlooking the facts. Consider an FBI intelligence assessment from last August, obtained by Foreign Policy’s Jana Winter and Sharon Weinberger. The report warns of a new and highly dangerous domestic terrorist threat: “Black Identity Extremists.” Notwithstanding that “BIE violence has been rare over the past 20 years,” the FBI proclaimed that it had “high confidence”—defined as “high quality information from multiple sources”—that “premeditated attacks upon law enforcement” by armed African-American activists were 80% to 95% likely to occur over the following year.

Six months later no such attacks have taken place. But in a Dec. 12 raid on the Dallas home of a black activist, FBI agents did seize two firearms, along with a copy of Robert F. Williams’s well-known book “Negroes With Guns,” first published in 1962. According to a Jan. 30 report in Foreign Policy, the activist stands accused of unlawful possession of a firearm, given a previous misdemeanor conviction for domestic assault….