Washington (CNN) Even before the debris from the Paris terrorist attacks was swept away, politicians began sounding the alarm that Syrian refugees could be a national security threat to the United States. The issue has dominated the U.S. political conversation during the week since gunmen and suicide bombers terrorized Paris on a Friday night.

All Republican presidential candidates called on President Barack Obama to renege on his pledge to admit 10,000 refugees fleeing Syria's brutal civil war into the U.S. and argued instead for a full stop, fearing terrorists could infiltrate their ranks.

Thirty-one governors have declared Syrian refugees unwelcome in their states and on Thursday the House passed a bill to bar refugees from Syria and Iraq from entering the U.S. Nearly 50 Democrats joined 242 Republicans to pass the bill, which the White House has threatened to veto. Sen. Ted Cruz, a Republican presidential candidate, suggested the U.S. only accept Christian refugees . Ben Carson, another candidate, likened refugees to "rabid dogs" threatening the neighborhood.

But those responses ignore one very important fact: the refugee program is quite simply the toughest way for a foreigner to legally enter the United States. There are other security gaps that would be easier for would-be terrorists to exploit.

Were any of the Paris attackers refugees?

As of now, none of the Paris attackers have been confirmed as having entered Europe as refugees.

In fact, most of the Paris attackers were European citizens born in France or Belgium. Two of them appear to have entered Europe through Greece although it doesn't appear that they came in through a refugee program.

A Syrian passport found next to one of the attackers' bodies stoked fears that the man had been a refugee. That has not yet been confirmed, although top European officials have suggested the passport was doctored , which raises its own set of questions, but does not confirm the suspected attacker was a refugee. Others have definitively been shown to be European citizens

Perhaps more importantly, the European refugee admission system is dramatically different from the U.S. system for Syrians, in large part because the U.S. is geographically separated from Syria. The U.S. has the opportunity to do far more vetting before refugees arrive on their shores.

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How does a refugee get into the U.S.?

Refugees must undergo an 18- to 24-month screening process, minimum, that the United Nations' refugee arm oversees. And that's before individual countries even begin to consider a refugee's application and conduct their own additional interviews and background checks.

The screening process generally includes multiple interviews, background checks and an extensive cross-referencing process that tests refugee's stories against others and accounts from sources on the ground in their home country.

Throughout that process, U.N. officials and local government officials in temporary host countries like Turkey, Jordan and Lebanon look to determine the legitimacy of asylum seekers' claims and ensure that they meet the criteria of a refugee, including that they are not and have not been involved in any fighting or terrorist activities.

Refugees also have their retinas scanned and have their fingerprints lifted.

Christopher Boian, a spokesman for the United Nations High Commission for Refugees, called the process "stringent" and "long and complex."

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"If at any stage in that process there is ever the slightest shadow of a doubt or the slightest whisper of suspicion, they are removed from the process. That is that," Boian said.

"The very, very few Syrian refugees who are accepted and referred for consideration for resettlement in another country -- there simply is no more closely scrutinized population on earth these days," he added.

That's because other countries have so far pledged to resettle just 159,000 of the more than 4 million Syrian refugees -- setting an extremely high bar for resettlement.

And refugees aren't automatically considered for resettlement: only the most vulnerable refugees -- such as torture victims, female heads of household, people with serious medical conditions and other especially vulnerable groups.

So after they go through that process by the U.N., the U.S. does an additional screening?

That's right. After a rigorous screening process and several interviews carried out by the U.N. refugee agency, refugees the U.S. agrees to consider for resettlement have to undergo an additional interview, medical evaluation and security screening.

According to one U.S. government official, there's an additional layer of vetting that's specific to Syrian applicants, including special briefings for interviewers and information from the U.S. intelligence community.

The security screening involves checks against several government agencies' databases and terrorist watch lists using biographic and biometric information. It's a process Mark Toner, a State Department spokesman, recently called "the most stringent security process for anyone entering the United States."

And Syrian refugees get an additional, more targeted layer of screening involving the U.S. Intelligence agency, according to a government official.

Sounds pretty rigorous. How does the refugee process stack up to other ways of getting into the U.S.?

The refugee program is simply the toughest way for any foreigner to enter the U.S. legally.

For most people, getting a tourist visa to enter the United States is much easier, but still requires an in-person interview and involves a typical background check. The process takes anywhere from a few days to a couple months.

But there's an even easier way to get into the U.S. if you're a citizen of one of 38 mostly European countries, including France and Belgium.

Travelers from those countries don't even need to first apply for a visa to get into the United States. They buy a ticket, grab their passport, and undergo the usual screening from U.S. customs officials when they land in the U.S. They are still checked against security databases before they get on the plane and upon arrival.

The fact that most of the Paris attack suspects were European citizens who would have had access to the visa waiver program is setting off some alarm bells. At least one of the eight Paris attackers likely would have been able to travel to the U.S. under the visa waiver program , U.S. national security officials told CNN Friday.

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As a sign that the Obama administration agrees that there are gaps that need closing, one of the U.S. officials said, in the coming days the administration expects to announce plans for additional steps to be taken with European countries that participate in the visa waiver program.

Sen. Angus King, an independent from Maine who sits on the intelligence committee, said it "would be much harder" for a terrorist to get into the country through the refugee program than with a passport from one of the 38 countries in the visa waiver program.

"(The refugee process) would take 18 months to two years. Under the visa waiver program, it could take 24 hours," King told CNN in a phone interview. "The target of our work should be strengthening the visa waiver program."

"We do need to pay attention to whether the terrorists could infiltrate the refugee flow. I don't think it's something we should ignore, but the amount of vetting that goes on there already is very through," King added.

So is that program getting strengthened?

A bipartisan proposal to do just that is gaining momentum on Capitol Hill.

Noting that 20 million people each year use the visa waiver program to visit the United States, Sen. Dianne Feinstein, D-California, said in a Thursday news conference that a bill she is proposing with Sen. Jeff Flake, R-Arizona, would help guard against terrorists trying to exploit the program.

"Terrorists could exploit the program, could go from France to Syria, as 2,000 fighters have done, come back to France, use the visa waiver program and without further scrutiny come into the United States," said Feinstein, a senior member of the intelligence committee.

The Feinstein-Flake bill, which is set to be formally introduced after Thanksgiving, would keep foreigners who've traveled to Syria or Iraq in the last five years from using the visa waiver program. It would also mandate fingerprinting for all travelers entering the U.S. from visa waiver countries and requires all foreigners from those countries to have a modern passport that has an embedded e-chip that is more secure and includes an individual's biometric information and other data.

Flake, the bill's Republican sponsor, told reporters Thursday the refugee program could be strengthened to include better tracking of refugees once they arrive in the country, but said touted the rigorous process as something that shouldn't be a source of concern.

"On the front end, it is a very thorough vetting that they get. So of all the things that we ought to be concerned about, that is not at the top of the list," he said.