Professor Theoharis became a consultant to the committee, which in 1975 and 1976 investigated the legality of the F.B.I., the C.I.A. and the National Security Agency’s intelligence operations. He did research in the archives of several presidential libraries, including those of Truman, Eisenhower and Lyndon B. Johnson, on the classified material the F.B.I. sent to presidents.
“They have access to F.B.I. records, unrestricted access,” he told Ms. Medsger and Ms. Hamilton, referring to the Church Committee and its counterpart in the House, led by Representative Otis Pike, a New York Democrat. “And it’s a different ballgame.”
And it was for Professor Theoharis as well. He deployed FOIA, which had been strengthened by Congress in 1974, to plumb Hoover and his top aides’ sensitive “Official and Confidential” files, along with those designated “Do Not File,” which were kept from the F.B.I.’s central records, presumably safe from being disclosed.
“That absurd “Do Not File’ file was one of the things that Athan drilled down on,” Professor Gage said, “and he got a lot of information that way.”
Professor Theoharis wrote numerous books about the F.B.I., among them “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’s commentary.
Reviewing “The Boss” in The New York Times, 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.”
Professor Theoharis thought that the portrait of Hoover as a homosexual cross-dresser that emerged in Anthony Summers’s 1993 book, “Official and Confidential: The Secret Life of J. Edgar Hoover,” was a distraction from the seriousness of Hoover’s unchecked authority.