The department that the U.S. Attorney General heads is called the "Department of Justice," and not the "Department of Security."
Senator Brownback said that the first priority of the Attorney General is protection of the security of the United States.
The Attorney General is in part a prosecutor. But the Attorney General also has the duty to see that justice is done. The precise manner in which these two duties should be combined is subject to disagreement. But few informed observers (other than Senator Brownback) believe that the duty of the Attorney General to promote security takes absolute precedence over every other duty.