United States officials fear people with ties to the Islamic State might have traveled to the United States using fraudulent Syrian passports, including some the terrorist organization printed itself, according to a recent Homeland Security Intelligence report obtained by ABC News.

The intelligence report, ABC News reported, says that ISIS has theoretically had the ability to print fake passports for more than 17 months, following their capture of the Syrian city of Deir ez-Zor. The group could have theoretically gained access to the city’s passport office, the report indicated, and its “boxes of blank passports” and at least one passport printing machine. The Islamic State’s longstanding capital is the Syrian city of Raqqa, home to another passport office.

“Since more than 17 months [have] passed since Raqqa and Deir ez-Zour fell to ISIS, it is possible that individuals from Syria with passports ‘issued’ in these ISIS controlled cities or who had passport blanks, may have traveled to the U.S.,” ABC News quoted the report as saying.

While not specifically mentioning the report, FBI Director James Comey testified before Capitol Hill lawmakers on Wednesday and mentioned concerns about ISIS forging travel documents.

“The intelligence community is concerned that they have the ability, the capability, to manufacture fraudulent passports, which is a concern in any setting,” Comey told lawmakers.

It’s been widely reported that at least one of the men who carried out last month’s terrorist attacks in Paris had a fake Syrian passport. ISIS has claimed responsibility for those attacks.