Sen. Dianne FeinsteinDianne FeinsteinA guide to the committees: Senate Dem: Trump's China trademark looks like a quid pro quo Senate advances Trump's Commerce pick MORE (D-Calif.) said Sunday that the U.S. visa waiver program poses a threat to national security by making it easier for terrorists to enter the United States.

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"The visa waiver program is the Achilles' heel of America," Feinstein said on CNN's "State of the Union," urging Congress to examine the program.

"[Terrorists] could come back from training, they go to a visa waiver country and they could come into this country."

The California Democrat was responding to the attacks in Paris last week in which Muslim extremists targeted satirical newspaper Charlie Hebdo and a Jewish deli. Seventeen people died.

Critics have said France was lax in tracking the three men behind the attacks, all of whom were killed last week. Feinstein replied that it's "difficult to ferret someone out" as they travel country to country.

"There are stolen travel documents in large numbers," she said. "They could pick up a false passport. … We have a big problem there."

Defenders of the U.S. visa waiver program said it has heightened U.S. security by increasing intelligence sharing between countries.