A 17-page US Homeland Security Investigations Intelligence Report, issued to law enforcement last week, said ISIS likely has been able to print legitimate-looking Syrian passports since taking over the city of Deir ez-Zour, home to a passport office with 'boxes of blank passports' and a passport printing machine.

Another passport office was located in Raqqa, Syria, which has long been Islamic State in Iraq and Syria's de facto capital.

The report notes that the primary source for the information was rated at 'moderate confidence', the second-highest rating given for source assessments.

Testifying before lawmakers on Wednesday, FBI Director James Comey first publicly talked about the nation's top security officials' anxiety over the problem.

Fake Syrian passports have already been discovered in Europe, most notably two used by suicide bombers in the horrific terrorist attack on Paris last month. The two men are believed to have slipped into Europe with a flood of Syrian refugees fleeing the violence in their homeland.

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