Congressional Republicans sounded an alarm Wednesday over rules allowing millions of Europeans to travel to the United States without a visa, warning that jihadists could exploit weak links in the program.
The Islamic State and Al-Qaeda extremist groups "have incurred great losses in Syria and Iraq," Michael McCaul, chairman of the House Homeland Security Committee, told a hearing on the security of the US Visa Waiver Program (VWP).
"Yet as the territory under their control shrinks, we are seeing an exodus of foreign fighters returning to their homelands," he said.
House Republican Mike Gallagher pointed to deadly attacks in Paris, Brussels, Nice and Berlin as examples of why the US should increase its vigilance.
"The majority of these attackers were European citizens with valid passports, so it is easy to imagine any one of them gaining access to this country through a valid visa or through the Visa Waiver Program," said Gallagher, who heads the Task Force on Denying Terrorists Entry to the United States.
"While there are numerous benefits to our country that stem from our welcoming immigration system -- like tourism, trade, and business -- we should never cease to examine our processes through the lens of a terrorist in search of potential gaps."
Millions of travelers from 38 wealthy countries, including 30 European nations, currently benefit from the exemption, which allows them to come to the United States for 90 days without having to go through a lengthy visa process.
Only a simple Electronic System for Travel Authorization (ESTA) form need be filled out online beforehand.
But since the attacks that bloodied France in 2015, committed by or with the help of jihadists bearing French or Belgian passports, many lawmakers see the exemption as a dangerous security flaw.
In December 2015, a few weeks after the Paris attacks, Congress tightened VWP rules, banning people from using the exemption if they had traveled after March 2011 to any of seven countries deemed to be risks: Iran, Iraq, Libya, Somalia, Sudan, Syria and Yemen.
It also barred citizens from VWP countries who had dual nationalities from Iran, Iraq, Sudan or Syria -- for example a Frenchman with Syrian citizenship -- from taking advantage of the exemption.
Since last October, more than 16,000 people have had their ESTA request rejected according to these criteria, said John Wagner, a deputy commissioner for US Customs and Border Protection.
The individuals affected are not automatically denied US entry, but must apply for a visa from a US consulate where comprehensive security measures are undertaken, such as scanning fingerprints and conducting interviews.