But no amount of border enforcement will have an impact on ''visa overstays," because they don't cross the border illegally in the first place, said Deborah Meyers, a senior analyst at the Migration Policy Institute, a Washington think tank.

''The overstay component has been overlooked," Meyers said. ''It's very important to make sure you're actually trying to solve the whole problem, not just the most visible parts of the problem. . . . From a security perspective, in some ways the overstay population is a bigger threat."

A potential terrorist who can obtain a valid visa is more likely to enter the United States legally than to risk apprehension by trying to sneak across the border with Mexico, Meyers said. Two of the 19 Sept. 11 hijackers were in the United States on expired visas. At least six others had otherwise violated immigration laws but were nonetheless able to stay in the United States, according to the 9/11 Commission report.
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