U.S. Customs and Border Protection Commissioner R. Gil Kerlikowske sits under an image of New York's Ground Zero in his office in Washington. For him, it serves as a daily reminder of the security threats that have shaped his agency. Kainaz Amaria/NPR hide caption

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Maria Guadelupe Guereca Betancourt, a resident of Juarez, Mexico, lost her son Sergio, 15, when he was shot under the black bridge that spans the border from El Paso, Texas, to Juarez. Kainaz Amaria/NPR hide caption

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Family members huddle at the fence to talk to loved ones living across the border. Kainaz Amaria/NPR hide caption

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Parts of the fence along the U.S.-Mexico border might stop vehicles, but they don't keep out those making the journey on foot. Kainaz Amaria/NPR hide caption

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Columbus, N.M., was raided by Pancho Villa in 1916 and by federal agents in 2011. Kainaz Amaria/NPR hide caption

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Workers arrive at an assembly plant located along the border. Kainaz Amaria/NPR hide caption

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Dob Cunningham (left) and his friend Larry Johnson look over the edge of Cunningham's 800-acre ranch in Quemado, Texas. Kainaz Amaria/NPR hide caption

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