Athan Theoharis, a preeminent historian of the FBI, was a master at unlocking the secrets of J. Edgar Hoover’s FBI. His 23 books and scores of articles, most of them reports on Hoover’s massive surveillance and his secret illegal operations that were designed to destroy individuals and organizations whose opinions he disliked, provide the most extensive record of Hoover’s half-century reign as FBI director.

Theoharis died July 3 in Syracuse after complications from surgery. From 1969 until his retirement in 2006 he was a professor at Marquette University, to whose library’s archives he donated his thousands of FBI files.

Nearly as important as the deep knowledge and insights Theoharis developed about the nearly half-century that Hoover was FBI director, from 1924 until his death in 1973, were the skills Theoharis developed to unlock Hoover’s secrets, skills he shared with generations of students and journalists.

Unlocking Hoover’s files was a difficult task. He intended for all of his files to remain secret forever and went to great lengths to shield them beneath layers of secrecy. He especially feared, for good reason, that the revelation of certain files would be extremely damaging to his reputation and went to even greater extremes to guarantee that these files would be secret forever. The file system he created consisted of mazes within mazes. He used tricky nomenclature and lies to hide his files. The names of some parts of the system were especially strange. For instance, Theoharis discovered that one large segment of files carried the label “Do Not File.” It contained files about particularly cruel operations, all of them illegal. Buried under that title Theoharis found Hoover’s Obscene File and Sex Deviates File, records of the intimate lives of members of Congress and other prominent people that he maintained for use as blackmail when a perceived need arose.

Hoover renamed some operations periodically. For instance, fearful that the Custodial Detention Index, established in 1939, might be discovered, he renamed it the Security Index in 1943 and renamed it again in 1971 as the Administrative Index. In his mind, this made it possible for him to say a given program did not exist if, say, an attorney general inquired about it. Actually, it still existed—under a new name but with the same ongoing purpose: to secretly detain without due process during a national emergency Americans he deemed subversive.

Originally, Theoharis focused his research on Cold War history. His first book, published in 1969, was Anatomy of Anti-Communism. His move from an emphasis on Cold War history to FBI history started inadvertently with an article he wrote for The Nation in June 1971: “Thirty Years of Wire Tapping.”

In January 1975 both houses of Congress took a historic step. The legislature had never exercised oversight of intelligence agencies, but now the Senate and House established committees to conduct the first congressional investigations of the FBI and all intelligence agencies. Because Theoharis’s 1971 Nation article demonstrated his competence at researching documents in presidential libraries, including files that documented how some presidents and the FBI director used each other for their political interests, he was hired as a consultant to the Senate Select Committee to Study Governmental Operations with Respect to Intelligence Activities, known as the Church Committee for its chairman, Senator Frank Church, Democrat from Idaho.

The first documentary evidence of Hoover’s extensive surveillance of activists and other illegal operations had come from files stolen from the Media, Penn., FBI office in March 1971. A group of anti-war activists, who called themselves the Citizens Commission to Investigate the FBI, risked decades in prison in order to get evidence for the public of whether Hoover was suppressing dissent. To their surprise, they confirmed that and much, much more.

A single file, a mere routing slip, turned out to be the most important file removed from the Media office. In its upper right corner was a mysterious term: COINTELPRO—from Counter Iintelligence Program. What that term stood for was so shocking to the public that it compelled Congress to investigate the FBI for the first time.

At first, the FBI refused to explain what the term meant. Finally, as the result of a successful lawsuit brought by NBC reporter Carl Stern, the FBI was ordered by a judge to release the founding papers of those operations, still widely considered today to be the worst of the worst of Hoover’s operations. They ranged from crude to cruel to murderous. The FBI’s plot, conducted with the Chicago police, to kill Black Panther Fred Hampton—the story told in the recent film Judas and the Black Messiah—was a COINTELPRO operation.

Americans were astonished by the COINTELPRO operations. So was Theoharis. As a consultant to the Church Committee, he helped it reveal important FBI operations. It was then that he changed the direction of his scholarly research from the Cold War to the FBI. The voluminous research he did from then on was, in effect, a sustained expansion of the historic record of FBI abuse created by the Church Committee. His FBI research eventually led him to think that the Red Scare that took place during the Cold War would have more appropriately been called Hooverism rather than McCarthyism.

One of Theoharis’s most alarming discoveries was the existence of the American Legion Contact Program. This program, like Hoover’s national Black surveillance program that was revealed in the Media files, resembled the massive surveillance conducted by the Stasi, East Germany’s secret police. Through a secret formal agreement with the American Legion, 100,880 members of the organization’s 16,700 posts regularly reported information from 1940 to 1966 to regional FBI officials about their fellow citizens. At first, the emphasis was on spying on people who worked in industrial plants. Later, it expanded into general political spying. Theoharis revealed this program in an article he wrote in 1985 in Political Science Quarterly.

In a 2013 interview with Johanna Hamilton and this writer for a film and book about the 1971 FBI break-in, Theoharis reviewed some of the cruel history he had helped uncover. He hoped it would not be repeated, but he feared it already had been in the immediate aftermath of 9/11 with the passage of the Patriot Act, which made possible abuses by intelligence agencies.

He told us that after assessing many thousands of files that documented Hoover’s FBI operations, he had concluded that “the FBI gathered no information that had anything to do with finding out, say, that Joe Smith is going to bomb the Capitol, something that could have been stopped…. I can think of no crime that was stopped by information gained, for instance, during COINTELPRO and COINTELPROlike operations.”

Even more striking was his conclusion that Hoover had destroyed what was supposed to be the essential purpose of the FBI: its law enforcement capacity. “I know of no case,” said Theoharis, “where there was a benefit to society.… It was harass and destroy rather than investigate, prosecute and convict. Hoover took the law into his own hands. It had nothing to do with law enforcement. He, the person in charge of law enforcement, created a culture of lawlessness.”