The FBI pretty much was incorruptible until they were forced to lower their standards. After that, they went downhill.
Actually when Truman created the CIA, the FBI started to take short cuts in procedures and citizens rights to obtain information and arrest criminals, especially during the Red Scare. From 19 50s to the 1970s, they were breaking all kinds of civil laws, it stands to reason that is where a lot of corrupt agents in the Bureau started to go sway. That Panama Unit in Texas with the Sheriffs was actually financed by a secret FBI account that was used to conduct FBI operations in foreign countries, notably Latin America, mainly to keep tabs on foreign operatives that frequent the US.
