refugee terrorists?

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.

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.

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."