The Obama administration says it's being prudent.

“The security of the United States homeland comes first,” White House spokesman Josh Earnest said. “And that’s what’s going to guide our decision-making process, and that’s why -- that, frankly, is why it’s not possible for the United States to, overnight, ramp up the number of refugees that are admitted to this country.”

‘Sending Fighters Into The Masses’

The administration’s argument it cannot do more because it’s hamstrung by national security obligations plays directly into politics. By law, the president sets the number of refugees, but Congress appropriates the funds necessary to cover them. Republicans already have introduced legislation to block Obama’s decision to let in more refugees, saying it could result in a “federally funded jihadi pipeline.”

“Members of ISIS have said that they are sending fighters into the masses seeking refuge,” said an aide for House Judiciary Chairman Bob Goodlatte, R-Va. Goodlatte was set to host a hearing on the crisis’ security impact on Wednesday, but it has been postponed.

Republicans cite February testimony from a House Homeland Security Committee hearing in which National Counterterrorism Center Director Nicholas Rasmussen said Syrians trying to come to the U.S. are “clearly a population of concern.” At the same hearing, Michael Steinbach, the assistant director of the FBI’s Counterterrorism Division, said of the security clearance process, "You have to have information to vet.” NCTC and FBI officials declined to clarify the comments.

It’s not easy to pin down the basis for the national security concern cited by U.S. lawmakers and administration officials. For nearly a month, Defense One pinballed between some 25 sources at the Capitol, the White House, the NSC, the Office of the Director of National Intelligence, the National Counterterrorism Center, the FBI, DHS, the U.S. Citizenship and Immigration Service, the State Department, the Defense Department, the Defense Intelligence Agency and the CIA.

ODNI directed Defense One to four separate DHS agencies. NCTC ultimately directed queries to the FBI and DHS. The FBI also deferred to DHS headquarters. The CIA said the FBI would monitor refugees already in the U.S., but that DHS had the lead for questions on the Syrian refugee crisis. The State Department, Defense Department, and DIA referred Defense One to DHS and UNHCR.

Initially, however, Joseph Holstead of DHS’s citizenship and immigration service said the question was more appropriately directed to the State Department or the NSC – both of which sent Defense One back to DHS. Later, Holstead said the department couldn’t release the information without a Freedom of Information Act request. Defense One filed its FOIA on Sept. 21.

NSC officials did not respond to multiple requests for comment regarding the basis for their national security concerns, nor whether it requires a FOIA request.

In lieu of data, the DHS official ultimately gave this assessment: “We are not aware of any trends in general or with respect to Syrians.”

But several officials said the question may be difficult to answer because there is no quantifiable basis for the security worry that’s invoked to defend U.S. reluctance to let in more Syrian refugees. No U.S. official could, or would, point to any cases of Syrian refugees posing a threat.

‘Last Resort’

Wanting to do more for the refugees would seem to cut across the aisle. So too does maintaining strict requirements in the name of national security, despite the lack of data.

"Whenever there is an influx of refugees from any country, and especially a war zone like Syria with a glut of terrorists, there are going to be serious security concerns,” House Intelligence Committee Ranking Member Adam Schiff, D-Calif. told Defense One. “These security issues do not condemn us to inaction in the face of an unprecedented humanitarian crisis, however.”

Senate Intelligence Committee Chairman Richard Burr, R-N.C., asked rhetorically, “Do we have a reason to look at Syrians and say they’ve been involved in terrorism? Yeah!”

“But probably the larger thing is the intent expressed by ISIL and others to say we need to get people in this chain.” There is no evidence of that happening yet, however, he acknowledged. “I am assured, and by everything I’ve checked, that we are as thorough today as we probably need to be and probably can be.” On Tuesday, he told Defense One the national security concerns are “probably based more on what we don’t know than what we do know.”

Sen. Chris Murphy, D-Conn., recently visited refugee camps in the Middle East. “Listen, national security comes first and so I don’t want to shortcircuit the vetting process,” he said, “But there are plenty of people in the those camps who pose no security risk to the U.S. — we always give priority to children, to people who are infirm and fragile situations.”

The politicization of the crisis has carried over to the 2016 campaign trail. Former Maryland Gov. Martin O’Malley, and now former Secretary of State Hillary Clinton, have called for the U.S. to let in as many as 65,000 Syrian refugees.

Donald Trump said on Fox News Saturday that welcoming Syrian refugees could result in “one of the great military coups of all time if they send them to our country.”

Senate Foreign Relations Committee ranking member Sen. Ben Cardin, D-Md., told Defense One he didn’t know the basis for the cited national security concerns, “because I don’t think there is one.”

As she headed to a recent meeting with European ambassadors to discuss the refugee crisis, Sen. Jeanne Shaheen, D-N.H., said simply, “I think they’re based on September 11.”

Many Syrian refugees resisted resettlement until lately, holding out hope they’d be able to return home, UNHCR’s Yungk said. “People think of resettlement as this golden door for everybody,” he said, “but for many people it’s a last resort.”

As for the Islamic State’s latest response to the exodus, instead of encouraging Syrians to infiltrate the refugee migration to the West, the terrorist group is threatening and cajoling people fleeing the region to stay and join the caliphate.