Final answer:
The United States Department of Justice operates the National Institute of Justice as its research arm, focusing on the development and evaluation of crime control and justice issues.
Step-by-step explanation:
The federal department that operates the National Institute of Justice (NIJ) as its research arm is the United States Department of Justice. The department's mission, among other responsibilities, is to defend the interests of the United States, ensure public safety, provide federal leadership in preventing and controlling crime, seek justice for those guilty of unlawful behavior, and ensure fair and impartial administration of justice for all Americans. The NIJ plays a crucial role within the Department of Justice, serving as the agency that is responsible for the research, development, and evaluation of crime control and justice issues.