The Border and Transportation Security directorate, the largest of the directorates in the department, brought together several agencies from such departments as the Treasury Department, the Department of Justice, the Department of Transportation, and the Department of Agriculture. The mission of this directorate is to secure the borders and transportation systems of the United States and to enforce immigration laws. This directorate consists of four main agencies: the Transportation Security Administration, Customs and Border Protection, Immigration and Customs Enforcement, and the Federal Law Enforcement Training Center.