Migrants and refugees, many of them Syrian, rest on the ground as they wait to enter a registration camp after crossing the Greek-Macedonian border on Nov. 18. Migrants and refugees, many of them Syrian, rest on the ground as they wait to enter a registration camp after crossing the Greek-Macedonian border on Nov. 18. Federal agents are reinvestigating the backgrounds of dozens of Syrian refugees already in the United States after discovering a lapse in vetting that allowed some who had potentially negative information in their files to enter the country, two U.S. law enforcement officials said.