President Obama's administration contends that refugees are not the true source of U.S. security concerns. Jeh Johnson, the secretary of Homeland Security, tells NPR that the real concern may be a person traveling as an ordinary tourist from Europe.

Johnson's department is tightening the visa waiver program, under which visitors from 38 countries, including much of Europe, may travel to the United States without applying for visas.

Administration and congressional leaders in both parties have called attention to the visa waiver program, noting that most suspects identified in the recent attacks in Paris were not Syrian refugees, but legal residents of European countries.

Among other things, the U.S. will now deny visa-free travel to those who have previously visited "terrorist safe havens" such as Syria or Iraq. Johnson says the Obama administration wants to work with Congress on legislation that will further strengthen the routine electronic screening of those who are still able to travel without visas.

But the administration still resists a House-passed measure that would make it more difficult for refugees to enter the United States.

"What we want to do is work with Congress in ways that are effective to improve homeland security," Johnson said. "The refugee program as it is currently exists is probably the most thorough, multilayered, time-consuming way for anyone to cross our borders, to come into this country."

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