Europol Director Rob Wainwright warned that the European Union is facing a growing threat of Islamist fighters returning from Syria, and says that as many as 5,000 Europeans are still fighting in Syria, Antiwar reports.

This has been a long-standing concern, as those fighters will eventually return home with ties to international jihadist movements, and an eye toward attacks in the Western world.

Wainright warned Europe faces a “capability gap” in dealing with the returning Islamists, and that the risk of sleeper cells was more than at any time since 9/11.

Europeans have flocked to Syria, mostly to join the Islamic State, in numbers unprecedented, and IS and the other groups have been able to absorb them with relative ease. Some 1,000 jihadists head to Syria and Iraq from around the world during any given month now, as the U.S.-led war makes it even easier for IS to recruit from abroad.