Correct, but is it codified law as to what their mission is supposed to be? Law enforcement means one thing verses National Security which implies a whole other set of parameters. One's that are not delegated (codified by law) to this branch of government?
I may be wrong, but I do not believe their mission is codified by law. Since their inception, their duties have encompassed criminal justice and (national security) intelligence matters, and has flowed seamlessly between the two. If memory serves, their major expansion into the area of “national security” occurred during World War II, chasing saboteurs and espionage agents, prior to, and subsequently competing with the OSS and its successor, the CIA.