Department of Justice

The mission of the Department of Justice is:

To enforce the law and defend the interests of the United States according to the law; to ensure public safety against threats foreign and domestic; to provide federal leadership in preventing and controlling crime; to seek just punishment for those guilty of unlawful behavior; and to ensure fair and impartial administration of justice for all Americans. 

Source: Department of Justice

It is headed by the United States Attorney General, a cabinet position nominated by the President and confirmed by the Senate. 

See also Attorney General Jeff Sessions.