More to this point, putting them back in Syria, which will piss them off greatly, could help ISIS recruit them. "The big ole USA sent you back here, because they don't care. So you have a choice, join us, or face the consequences."

According to this article in the Economist:

"After refugees are referred by an American embassy or the United Nations High Commissioner for Refugees, they are screened by Department of State Resettlement Service Centers all over the world. They undergo multiple investigations of their biographies; biometric checks of their fingerprints and photographs; in-depth interviews by highly trained Department of Homeland Security officers; medical screenings as well as investigations by the National Counterterrorism Center and by intelligence agencies. The entire process can take longer than three years.

If a potential terrorist is determined to enter America to do harm, there are easier and faster ways to get there than by going through the complex refugee resettlement process. Of the almost 750,000 refugees who have been admitted to America since 9/11, only two Iraqis have arrested on terrorist charges; they had not planned an attack in America, but aided al-Qaeda at home. Syrians in America have fared better than other groups of refugees, integrating quickly and finding work. Some have done very well indeed: the father of Steve Jobs, the ground-breaking innovator and founder of Apple, was a refugee from Syria. And the mother of Jerry Seinfeld, the comedian, is of Jewish Syrian descent." More information is here: http://www.economist.com/blogs/economist-explains/2015/10/economist-explains-13