<P> There are an estimated half a million illegal entries into the United States each year . Border Patrol activity is concentrated around border cities such as San Diego and El Paso which have extensive border fencing . This means that the flow of illegal aliens is diverted into rural mountainous and desert areas, leading to several hundred migrant deaths along the Mexico--U.S. border of those attempting to cross into the United States from Mexico illegally and vice versa . </P> <P> The Secure Fence Act of 2006 was passed providing for the construction of 700 miles (1,100 km) of high - security fencing . Attempts to complete the construction of the Mexico--United States barrier have been challenged by the Mexican government and various U.S.--based organizations . </P> <P> In January 2013, the Government Accountability Office released a report stating that the United States Border Patrol only intercepted sixty - one percent of individuals illegally crossing the border in 2011, which translated to 208,813 individuals not apprehended. 85,827 of the 208,813 would go on to illegally enter the United States, while the rest returned to Mexico and other Central American countries . The report also showed that the number of illegal border crossings has dropped . </P> <P> Statistics from the United States Border Patrol agency on apprehensions in the last 10 years in the border region show that 91 percent of the apprehensions made were illegal border crossings . Similarly, the U.S. Border Patrol (USBP) statistics show that 90 percent of the people apprehended along the border were from Mexico . The number of apprehensions along the border with Mexico has dramatically declined from 2001 to 2010 as the United States government increased the number border patrol agents and other facilities along the southwest sectors of the border . According to the Department of Homeland Security, there were about 10,000 agents operating along the border areas in 2004 which was increased to 20,500 agents in 2010 . The Department of Homeland Security has also installed new technological equipment at the border including car and air surveillance, surveillance cameras, thermal imaging systems, inspection equipment, and so forth . Illegal border crossing is not the sole cause of illegal immigration to the United States, there are also people from various countries legally entering the United States and overstaying beyond their visa status who are also considered illegal immigrants . In 2010, there were approximately 11 million immigrants living the United States that were not authorized . Of the 11 million, 6.1 million people were from Mexico which accounted for 55 percent of the total unauthorized immigrants . </P>

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