US politicians are calling for changes to a law that allows Europeans and other foreigners to enter the country without visas, citing fears that jihadists could exploit the rules to stage attacks on American soil.
The visa waiver program, which covers tourists from 38 countries, represents the "Achilles' heel of America," said Senator Dianne Feinstein, who is urging a tightening of the rules.
The attacks against the French satirical magazine Charlie Hebdo in Paris this month have renewed concerns in Washington that extremists with Western passports will slip into the US under the cover of the visa-free travel program.
Feinstein, former head of the Senate Intelligence Committee, is working on a bill to reform the rules that will be proposed soon, her aides said.
Other lawmakers also are eyeing changes to the law, including Candice Miller, a Republican from Michigan, who introduced a bill that would enable the Department of Homeland Security (DHS) to suspend countries from the program if they fail to provide key information on potential suspects.
The Paris attacks, carried out by men with French passports, and the growing number of Europeans volunteering to fight with jihadists in Syria and Iraq - an estimated 3,000 to 5,000 - offers a chilling reminder for Americans of dangerous terror plots.
Zacarias Moussaoui, known as the "20th hijacker" in the September 11, 2001 attacks, travelled to the US simply by presenting his French passport.
And Richard Reid, who tried to blow up a US-bound airliner in December 2001 with an explosive hidden in his shoe, needed no visa with his British passport.
However, the US government has dramatically revised procedures for the visa-free travel program since the 9/11 attacks.
Starting in 2008, passengers planning to travel under the visa waiver rules have to fill out a special form beforehand, known as the Electronic System for Travel Authorisation.
The electronic forms are an important tool, allowing US authorities to see well in advance who is trying to enter the US, said Christian Beckner, deputy director at the Centre for Cyber and Homeland Security at George Washington University.
