The terror attacks in Paris have prompted calls for more scrutiny of the US visa waiver program.
The waiver program, which allows travelers of certain countries to enter the United States for up to 90 days without a visa has been described as "the Achilles heel of America," by Sen. Dianne Feinstein.
"They can come back from training, they go through a visa waiver country, and they come into this country," Feinstein said, referring to potential terrorists.
"I think there are sleeper cells, not only in France, but certainly in other countries, and, yes, even in our own," said Feinstein, a former chair of the Senate Intelligence Committee.
"I think we need to take a look at the visa waiver program again, and see what we can do to prevent this kind of thing from happening, because I believe it will happen, if it hasn’t already."
The program has been an effective tool in promoting business and leisure travel to the US.
A congressional report said 19.1 million people used the program in 2012, representing 40% of all overseas visitors that year.
