Assuming you call the president and the pope "the president" and "the pope" when there's no name attached, and not "the President" and "the Pope," you should not be calling the attorney general and the U.S. attorney "the Attorney General" and "the U.S. Attorney." But doesn't the addition of "office" change things? It can. If you want to capitalize such an institution, fine: Attorney General's Office. U.S. Attorney's Office. If you don't, also fine: attorney general's office. U.S. attorney's office. But the caps need to match. If the pope isn't sacred, neither is the goddamned attorney general.
And if you're worried that a lowercased U.S. attorney will be mistaken for a simple American lawyer, random variance in capitalization is not the solution.