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SCOTT JAGOW: The President of Mexico arrives in Utah today for a three-day visit of western states. Vicente Fox will be learning first-hand about the growing frustration over illegal immigrants in the region. The Senate could pass its immigration bill this week. It includes plans to build a fence along the Mexican border. But a study out today suggests that might be kind of futile. Lisa Napoli reports.

LISA NAPOLI: The Pew Hispanic Center scoured immigration data. And it found nearly half of all the undocumented workers in the country actually entered the country legally. Millions overstayed their visas.

And nearly a half million crossed the border with papers like tourist visas or temporary work permitsa€¦and then violated admissions terms.

Jeff Passel of Pew says when you consider that 30 million people are given visas each year:

JEFF PASSEL: Even very small fractions of people overstaying mount up.

Ira Mellman of the Federation for Immigration Reform says the Pew study is proof a fence won’t work:

IRA MELLMAN: It’s not going to do anything to take away the reasons people come to this country and remain here illegally.

Mellman says cracking down on forged papers, and holding businesses that employ undocumented workers responsible, is the only way to fix the problem.

I’m Lisa Napoli for Marketplace.