In the wake of three back-to-back terrorist attacks in France that left 17 dead, the US Congress is being urged to take a second look at the visa-waiver program, which allows travelers from 38 countries, including Ireland, to enter the US without a visa.

The program is intended to promote tourism and business travel. However, with an estimated 3,000+ European citizens having recently traveled to Syria to train and fight with Jihadist groups, the program is now seen as a threat to national security and is falling under scrutiny.

The Charlie Hebdo terrorists were French citizens; as previously convicted terrorists they were on a “no-fly list” barring them from entering the US. But had they not been on the list, they would have been able to enter the US on the visa waiver program, a CBS interviewer said to Senator Dianne Feinstein, reputable California Democrat. “Is this a hole in the security net?” She asked.

“The visa waiver program is the Achilles heel of America,” Feinstein replied.

“They can come back from training, go through a visa waiver country, and come into this country. There are no-fly lists and there are terrorist lists, but [terrorists] are in the tens of thousands and even in the millions. So it’s difficult to ferret someone out."

She warned that jihadist groups have stolen travel documents that allow them to circumvent tracking efforts by U.S. and European intelligence: “There are stolen travel documents, and they can pick up a false passport, so we have a big problem there.”

“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.”

Feinstein says this issue is nothing new to her, and that she has been critical of the program since the Bush administration expanded it in 2007.

While increasing safety measures, dialing back the visa waiver program could be a potential blow to tourism and also make it harder for Irish citizens and others to enter the US for holidays. According to a congressional report, 19.1 million visitors entered the U.S. under this program in the 2012 fiscal year.

There are defenders of the visa-waiver program, however; former Department of Homeland Security (DHS) official Jayson Ahearn believes the program has built-in security that’s sufficient enough to stop terrorists without hindering trade and travel. He also sees its benefits aside from tourism:

"Many concerns about the visa waiver program are outdated and [critics] don't fully understand what the US does get as a result of having visa waiver agreements with 38 countries," he told CBS.

"[The 38 countries] all also engage in information sharing with the US. You begin with a pretty strong starting point."

In an interview with CBS, co-director of the Transnational Threats Project at the Center for Strategic International Studies, Tom Sanderson, cited three types of extremists who might try to exploit the visa waiver system: those looking to carry out suicide bombings, returning foreign fighters, and homegrown terrorists who did not travel outside of their home country for formal jihadist training. This last group has passports without warning signs.

"It's not a cakewalk to get into the United States," he said.

"Yet it is entirely reasonable to expect that some of those 3,000 fighters if they chose to get to the U.S. could do it with only a few hurdles," he added.

These issues are likely to emerge this week when the House votes on a bill to fund the Department of Homeland Security.

After France terrorist attacks, program allowing travelers from 38 countries to visit US without a visa is under scrutiny.Thinkstock