Pew study tracks the ebb and flow of undocumented immigrants in US

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An U.S. Customs and Border Protection bike patrol agent assists Mexican's being returned to Mexico after they were apprehended for entering the United States illegally June 2, 2010 in Nogales, Arizona. A fence which separates the cities of Nogales, Arizona and Nogales, Sonora Mexico is a frequent crossing point for people entering the United States illegally. During the 2009 fiscal year 540,865 undocumented immigrants were apprehended entering the United States illegally along the Mexican border, 241,000 of those were captured in the 262 mile stretch of the border known as the Tucson Sector. — An U.S. Customs and Border Protection bike patrol agent assists Mexican's being returned to Mexico after they were apprehended for entering the United States illegally June 2, 2010 in Nogales, Arizona.

Exactly how many unauthorized immigrants are living in the U.S.? It's a number that's difficult to pinpoint, but a new study by the Pew Hispanic Center seeks to give a more complete picture of the immigration flow in recent years.

Exactly how many unauthorized immigrants are living in the U.S.? It's a number that's difficult to pinpoint, but a new study by the Pew Hispanic Center seeks to give a more complete picture of the immigration flow in recent years.

Jeffrey Passel, lead author of the report, joins the show to explain.