Marquette University history professor Athan Theoharis was a persistent scourge of the FBI for its practice of violating Americans' civil liberties by spying on them. But he also had a technical respect for J. Edgar Hoover, the longtime FBI director who led those clandestine practices.

"Hoover was an insubordinate bureaucrat in charge of a lawless organization," Theoharis told Milwaukee Journal reporter Kevin Harrington in a 1993 interview. "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."

Theoharis countered the FBI's penchant for secrecy with dogged persistence, using each nugget of information he obtained to pry loose more nuggets. He pioneered the use of Freedom of Information Act requests in historical research, taught his many graduate students how to make such requests, and wrote a guide for scholars.

Theoharis died July 3 of pneumonia in Syracuse, New York, where he was living with his son George. He was 84.

"He cracked the FBI’s secret files partly because he knew the way was to figure out their filing systems, analyze them, request them, sue for them," his three children wrote in their memorial tribute to their father. 

Those files informed his many books, including "The Boss: J. Edgar Hoover and the Great American Inquisition" (1988), co-written with John Stuart Cox, and "From the Secret Files of J. Edgar Hoover" (1992). 

Theoharis' research proved or verified many episodes of FBI skullduggery, including multiple instances of Sen. Joseph McCarthy enlisting the FBI to seek sexual or political dirt on government officials and workers, even President Dwight D. Eisenhower himself. 

In a 1983 interview, Theoharis told Milwaukee Journal reporter James Rowen that his desire to unearth the truth about domestic government surveillance stemmed in part from the ethics his father taught him as a boy growing up in Milwaukee.

In 1975, Theoharis served as a consultant to the Senate Select Committee on Intelligence, also known as the Church Committee, investigating abuses by the FBI and other agencies. His book "Spying on Americans: Political Surveillance from Hoover to the Huston Plan" (1978) focused on abuses of domestic intelligence.