NEW YORK – Athan Theoharis, prominent historian whose research exposed civil liberties abuses at the Federal Bureau of Investigations (FBI) under J. Edgar Hoover’s leadership passed away on July 3 at his home in Syracuse, NY, the New York Times reported. He was 84.

The cause of death was pneumonia, his daughter Jeanne Theoharis told the Times.

“Beginning in the mid-1970s, Professor Theoharis, who taught history at Marquette University in Milwaukee, deftly used Freedom of Information Act requests to pry open the FBI’s deep well of secrets, including the extent to which Hoover compiled damning information on public officials and his cooperation with Senator Joseph McCarthy’s campaign against people he accused of being Communists,” the Times reported.

The documents “showed the extent of the agency’s break-ins and its illegal surveillance of left-wing organizations; its pursuit of allegations that President Dwight D. Eisenhower had extramarital affairs; and the use of the FBI by presidents of both parties for political purposes,” the Times reported, adding that “one of Professor Theoharis’ most alarming finds was a surveillance program forged by the FBI and the American Legion in 1940 that lasted until 1966” in which “the FBI used tens of thousands of the organization’s volunteers to report information about other citizens.”

“The goal of the program was to use Legionnaires, ‘who were highly motivated and who held pretty conservative views, who were going to act as the eyes and ears and expand the resources of the bureau beyond the agents,’ Professor Theoharis said in a joint interview in 2013 for the book The Burglary: The Discovery of J. Edgar Hoover’s Secret FBI by Betty Medsger, and 1971, a documentary directed by Johanna Hamilton,” the Times reported.

The 2014 book, “and the film, released the same year, dealt with the burglars who stole critical documents from an FBI office in Media, PA, which showed, among other things, active unlawful surveillance of Black, student and peace groups, and led to the revelation of Hoover’s secret Cointelpro program, begun in 1956, which spied on civil rights leaders, political organizers and suspected Communists,” the Times reported, noting that “before the creation of Cointelpro, the Legionnaires were ‘monitoring activities at defense plants, they were monitoring activities among ethnics within their community, they were monitoring activities of radical activists,’ Professor Theoharis said.”

His “strategic use of the Freedom of Information Act, or FOIA, enabled him to find pathways to documents through a purposely evasive filing system that Hoover had hoped no one would ever divine,” the Times reported.

“Hoover was an insubordinate bureaucrat in charge of a lawless organization,” Theoharis told The Milwaukee Journal Sentinel in 1993, “he was also a genius who could set up a system of illegal activities and a way to keep all documentation secret for many years,” the Times reported.

Beverly Gage, a history professor at Yale University who is finishing a biography of Hoover, said in a phone interview with the Times “that one of Professor Theoharis’ tactics had been to request ‘special agent in charge’ orders that showed which policies Hoover had wanted the FBI’s field offices to follow.”

“For me, these records gave me an institutional sense of the inner workings of the FBI,” Professor Gage told the Times, adding that “he figured out the key words to file the right FOIA requests.”

Theoharis “was an eager mentor to his graduate students and to historians on both the left and the right, helping them to navigate the FBI’s filing system,” the Times reported, adding that “Kenneth O’Reilly, a professor emeritus of history at the University of Alaska, Anchorage, was a graduate student of his in the mid-1970s whose dissertation was on the relationship between the FBI and the House Un-American Activities Committee, but after filing a FOIA request, he was told he would have to pay copying costs of several thousand dollars, and he almost dropped the subject.”

Theoharis told him “Forget about it. I know people. I know people in Chicago,” Professor O’Reilly said at Theoharis’ Zoom funeral, the Times reported, adding O’Reilly’s reply, “‘Forget about it’? ‘I know people’? For a brief moment, I thought, ‘My dissertation adviser sounds like a gangster.’”

Theoharis “had arranged a fund-raising dinner that paid O’Reilly’s costs,” the Times reported, adding that Theoharis “donated his voluminous trove of FBI papers to Marquette.”

“McCarthy’s files are there — my father took joy in that,” Jeanne Theoharis, a professor of political science at Brooklyn College, told the Times.

Athan George Theoharis was born on August 3, 1936, in Milwaukee, WI. His father, George, a Greek immigrant, and his mother, Adeline (Konop) Theoharis, ran a diner. At age 16, he began his studies at the University of Chicago, where he earned bachelor’s degrees in 1956 and 1957, a master’s degree in 1958, and a PhD in history in 1965, the Times reported, noting that Theoharis “taught history at what is now Texas A&M University, Wayne State University in Detroit and Staten Island Community College (now part of the College of Staten Island) before joining the Marquette faculty in 1969. He taught 20th-century American history there until his retirement in 2006.”

Theoharis’ early books include The Yalta Myths: An Issue in U.S. Politics, 1945-55, published in 1970, and Seeds of Repression: Harry S. Truman and the Origins of McCarthyism, published in 1971.

“In his research, he found documents dealing with wiretapping policy and the federal employee loyalty security program during the Truman administration, including records in the Truman library about the expansion of the FBI’s wiretapping authority,” the Times reported, adding that “his article Thirty Years of Wire Tapping, published in The Nation in 1971, brought him to the attention of the Senate Select Committee to Study Governmental Operations With Respect to Intelligence Activities, better known as the Church Committee for its chairman, the Idaho Democrat Frank Church.”

Theoharis “became a consultant to the committee, which in 1975 and 1976 investigated the legality of the FBI, the CIA and the National Security Agency’s intelligence operations,” the Times reported, noting that “he did research in the archives of several presidential libraries, including those of Truman, Eisenhower and Lyndon B. Johnson, on the classified material the FBI sent to presidents.”

Among his many books on the FBI are The Boss: J. Edgar Hoover and the American Inquisition” (1988, with John Stuart Cox) and From the Secret Files of J. Edgar Hoover (1991), “which reprinted agency memorandums accompanied by Professor Theoharis’ commentary,” the Times reported, adding that in a Times review of tThe Boss, Herbert Mitgang wrote: “Unlike some other recent Hoover biographers, the authors do not make apologies for the excesses of ‘The Boss.’ They have the goods on him.”

In addition to his daughter Jeanne, Theoharis is survived by another daughter, the Rev. Dr. Liz Theoharis, co-chairwoman of the Poor People’s Campaign; a son, George, a professor of educational leadership at Syracuse University; his sisters, Arhontisa and Zoe Theoharis; his brother, Theoharis Theoharis; and five grandchildren. His wife, Nancy (Artinian) Theoharis, a human-rights activist, died last year.