Once upon a time, President Richard Nixon turned the Department of Justice into his personal flunkies in various different conspiracies to break the law. Before it was over, his first two attorneys general wound up in legal trouble. A third attorney general and his deputy resigned rather than carry out the further demands of the president.
During the Watergate investigation, the Justice Department made sure that the White House was updated on every development, which White House Counsel John Dean then used to prepare the false testimony of the campaign and White House staffers involved, and otherwise keep Justice from learning the truth. It was the revelation of this situation that eventually produced the first Watergate special prosecutor, and the universal demand that he be replaced by another independent prosecutor after Nixon had him removed.