The Obama administration's main defense of its plans to accept thousands more Syrian refugees is how the system has worked in the past: the multi-layered vetting process, the years-long waits, the minuscule number of refugees who've turned out to have some sort of connection to terrorism.
Statistically and logically, it makes little sense to block Syrians fleeing persecution and war from making new homes in America, government officials and advocates say. Likewise, they say, if a terrorist group like ISIS were to launch an operation in the United States, they couldn't find a more cumbersome way to do it than through the refugee program.
But the terror attacks in Paris has spawned a backlash against President Obama's plan to bring an additional 10,000 Syrian refugees into the country. The House on Thursday passed a bill that would essentially block the refugees by requiring intelligence agencies to certify to Congress that each refugee doesn't pose a national security threat.
Unlike Europe, where hundreds of thousands of refugees are crossing from Syria and across the Middle East, the United States has the luxury of being a lot more choosy. It exercises that prerogative with a highly restrictive review that begins with referrals from the United Nations High Commissioner for Refugees and continues with examinations by the Department of Homeland Security, State Department, the FBI and federal intelligence agencies.
Since Sept. 11, 2001, nearly 780,000 refugees have gone through that process and been resettled in the United States, State Department data show. In an interview with NBC News this week, Secretary of State John Kerry said that only a dozen of those people had been arrested or deported for possible links to terrorism.
At the same time, U.S. intelligence officials believe the risk of Islamic terrorists using the refugee crisis to infiltrate the U.S. and carry out a terrorist attack is low, according to intelligence briefings reviewed by NBC News.
But critics point out that Syria is a special case.
ISIS has declared its intention to send fighters among the flood of refugees from Syria, and a Syrian passport was found near the body of one of the Paris suicide bombers.
Because Syria has been torn apart by civil war, the government is a shambles and refugees aren't as likely to have verifiable documents to prove they are fleeing persecution and war — and not planning to attack America.
That makes the usual vetting methods much more difficult, critics say.
"There's no way to establish a process that parallels the process they've done in other countries," said Tony Shaffer, a former Army National Reserve intelligence officer and a fellow at the London Center for Policy Research. "What we need to do is figure out an alternative process that gives you some level of confidence."
Obama administration officials say their procedures will measure up, and that the backlash against the refugee program is the result of the kind of fear mongering that terrorists want. The president has gone as far as mocking governors who say they don't want any of the 10,000 Syrian refugees in their states. And he has vowed to veto the restrictive bill that passed the House.
But House Speaker Paul Ryan said the measure "reflects our values. This reflects our responsibilities. And this is urgent."
The issue has divided Americans. An NBC News online poll found that 56 of respondents of all political affiliations opposed increasing the number of Syrian refugees into the United States.
Rochelle Davis, who researches Middle Eastern refugees at Georgetown University, said the governors and other opponents "don't know what they're talking about."
State officials have no power to keep refugees from being resettled within their borders, and if they try to block federal support-service money from reaching the refugees, they will leave the new residents even more vulnerable, Davis said.
"We as a country should have some trust in our intelligence services and processes we put in place to make that happen, to protect ourselves," Davis said.
Of course, there are bad people everywhere, and even administration officials acknowledge that they can't guarantee that no potential terrorists will make it into the country as a refugee.
Asked for details of the 12 refugees Kerry said had been arrested or deported since 2001, the State Department referred the question to the Department of Justice, which did not immediately respond Thursday.
The Migration Policy Institute did its own research, and said it found three resettled refugees who'd been arrested for planning terrorist activities. They included two Iraqis who plotted to send money and weapons from Kentucky to al Qaeda, a case that forced an overhaul of the refugee process — which had previously been criticized as too slow — when authorities learned that the men had been cleared to enter the United States despite having attacked American troops in their home country. The other case involved an Uzbek man who was convicted of trying to support a terrorist organization.
But the threat of homegrown terrorists is much more viable than refugees, officials and researchers say. As the Paris attacks illustrated, ISIS has inspired terrorists to attack the Western countries where they are citizens. U.S. intelligence officials are devoting an increasing amount of their work tracking Americans who have become radicalized by ISIS through social media.
It is also relatively easy for potential attackers who are not on a terror watch list to take advantage of lenient visa rules to travel to the United States from countries in Western Europe.
"Who we really need to fear if we're going to be scaremongering is ISIS cadres who have visa waivers," said Anne Speckhard, who researches the psychology of terrorists at Georgetown.
The State Department on Thursday, meanwhile, said that the visa waiver program was "useful" and "served a purpose" but left open the possibility for change after taking criticism from all sides.
And run-of-the-mill fraud is another concern. On Wednesday, Honduran officials arrested five Syrian nationals they said were trying to get to the United States with fake Greek passports.
Speckhard said she agreed that Syrian refugee background vetting will be more difficult. But she said she was confident that officials will be able to weed out those who are making up their stories.
"I think it's ridiculous to get hysterical about Syrian refugees," she said. "We should be hysterical for them."