WASHINGTON -- As Congress considers how to enhance national security in response to the Paris terrorist attacks, one idea gaining traction is to reform the visa waiver program.

Created in 1986, the visa waiver program allows citizens of 38 countries -- including most European nations, as well as Australia, South Korea, Japan and Singapore -- to visit the U.S. for up to 90 days without obtaining a visa or being interviewed at a U.S. embassy or consulate. According to the State Department, more than 19 million people come to the U.S. each year under this program, primarily for tourism or business purposes. In turn, those 38 countries allow U.S. citizens to visit without a visa.

Lawmakers now fear that terrorists could exploit the program to enter the U.S. All of the known assailants in the Paris attacks were citizens of France or Belgium, which are both visa waiver countries.

"If a terrorist is going to try to come into this country, they're much more likely to use loopholes in the visa waiver program to do it instead of waiting two years to go through the refugee screening process," Sen. Chuck Schumer (D-N.Y.) said last week.