"Statistics on apprehensions represent one of the few indicators available regarding illegal entry or presence in the United States. This Office of Immigration Statistics Fact Sheet provides information on recent trends in U.S. Border Patrol apprehensions and the gender, age, country of origin, and geographic location of persons apprehended during 2005 through 2008. Data beginning in 2005 were obtained from the Enforcement Case Tracking System (ENFORCE) of the Department of Homeland Security (DHS). For prior years, data were obtained from the Performance Analysis System (PAS) of DHS. Apprehension statistics measure the number of foreign nationals who are caught for being in the United States illegally. Persons apprehended are subject to removal from the United States for violating the Immigration and Nationality Act. The vast majority of apprehensions, occurring near U.S. borders shortly after an illegal entry, are made by the Border Patrol of U.S. Customs and Border Protection (CBP) of DHS. Apprehensions beyond U.S. borders, involving foreign nationals illegally present in the United States who may have entered without inspection (EWI) or entered legally but lost their legal status, are handled primarily by Immigration and Customs Enforcement (ICE) of DHS."
United States Department of Homeland Security: http://www.dhs.gov/