"Maintaining the security of U.S. borders is a fundamental responsibility of the federal government. Various border security missions are assigned to the Department of Homeland Security, the Department of Defense, and other federal agencies that work in cooperation with state, local, and tribal law enforcement agencies. The success of their efforts depends on the availability of reliable information on the nature of potential threats to border security. Given the extent of the land borders and the long coastlines of the United States and the number of individuals and vehicles crossing borders legitimately, the task of identifying law breakers within the overall threat environment is a major challenge. Law enforcement agencies obtain information from their usual sources--reports of crimes committed, tip-offs from informers, technical monitoring devices that now include unmanned aerial vehicles, and other sophisticated devices. In recent decades, and especially after 9/11, the potential for terrorists coming across the border as well as extensive narcotics trafficking have led policymakers to reach beyond law enforcement agencies to seek out information acquired by intelligence sources, including signals intelligence, imagery intelligence, and human agents."