When it first began to what it has become was exactly what they had feared ...

“Federal Bureau of Investigation: History The Beginning Of The F.b.i. President Theodore Roosevelt initially asked the U.S. Congress to create a federal detective force in 1907. Congress opposed President Roosevelt's idea on the official grounds of the long-cited public disdain for an all powerful federal law enforcement agency. However, unofficially, it was significant that in 1906 two members of Congress had been prosecuted for fraud, the investigation of which was conducted by the Justice Department using agents from another federal agency. As a result, many members of Congress were concerned about giving the executive branch of government more investigative power (power perhaps to conduct more investigations of those individuals in the legislative branch). Along with denying President Roosevelt's request, Congress passed legislation that prohibited the Justice Department from using investigators from other federal agencies. Not to be stopped by Congress, in 1908 President Roosevelt created the Bureau of Investigation by executive order and directed Attorney General Charles Bonaparte to develop the agency within the Department of Justice. Twenty permanent and eighteen temporary investigators were hired. The action on the part of President Roosevelt led to considerable political conflict and to many political battles between Congress and the president. The fear of Congress was that the Bureau of Investigation was going to act as a sort of secret police—and, in fact, these fears were quickly substantiated in 1909 when it was learned that agents from the bureau had regularly opened the mail of Senator Benjamin Till-man, one of the bureau's most vocal opponents.” https://law.jrank.org/pages/1191/Federal-Bureau-Investigation-History-beginning-F-B-I.html