Congress has the power to make laws. The executive branch has the power to act on those laws. If the Congress feels the executive branch is doing wrong, then the Congress can force impeachment hearings. Otherwise, they're pretty much up a creek. And they must depend upon some corroboration from someone within the executive branch. Or they could always make things up.
Or they can legislate away the FBI. Then, no more outside investigations of their corruptions. I think it fair that if they can police the executive, then the executive can police them. Checks and balances. Of course, they can all conspire together but that's another matter.