A general security guard stands as a Syrian bus driver carries the passports and departure cards of Syrian passengers who arrived at Lebanon's northern Tripoli port to board a passenger ferry to Turkey, Sept. 22, 2015. REUTERS/Jamal Saidi

The Islamic State group has likely obtained the materials necessary for making fake Syrian passports after taking hold of two key Syrian cities in the past two years, and now security officials say they fear militants with the terrorist group, aka ISIS, could slip into Europe or the U.S. with those documents, the Wall Street Journal reported Wednesday. The issue has been of heightened concern since it was learned that at least one of the individuals responsible for last month’s deadly attacks in Paris, which killed 130 people, registered as a refugee at the Greek island of Leros with a fake passport.

Without communication with the Syrian regime, identifying fraudulent documents has become increasingly difficult, and videos have sprung up online offering steps to obtain a Syrian passport and impersonate Syrians. At the same time, ISIS supporters have carried out a growing number of attacks outside their foothold in Syria and Iraq in recent months, stoking concerns that the group has become increasingly aggressive abroad.

“It’s enormously difficult to figure out,” Richard Barrett, a former senior U.K. counterterrorism official, told the Wall Street Journal. ISIS “can probably make them good enough to get them past someone who has faced 10,000 refugees.”

The European Union’s border agency, known as Frontex, has recently sent experts to entry points into Europe to help notice fraudulent passports, but there are only 10 such experts, a Frontex spokesperson said. Also, identifying a fake passport that was made using professional material and real passport books is difficult, the spokeswoman said, according to the Wall Street Journal.

It’s not just Syrian extremists who some fear have tapped into a market of fake Syrian passports. Because European governments grant asylum to Syrian refugees, a black market has emerged for fake Syrian documents among migrants and refugees from other countries as well. European authorities have arrested a number of refugees from elsewhere around the world with fake passports in recent weeks, though none have turned out to be security concerns.

Security analysts have said Syrians using fake passports would have a difficult time slipping into the U.S. as refugees, given the extensive security screenings people from that counry undergo. The U.S. has plans to accept some 10,000 Syrian refugees in 2016.