“If they are not already registered in the database, it’s nearly impossible,” says Dimitris Amountzias, police captain in charge of Moria, Greece’s main registration camp on the island of Lesbos.

At the camp, dozens of migrants and refugees queue to give their fingerprints, have a photo taken and be quizzed by agents from European border agency Frontex.

It’s a seemingly detailed security check, but jihadists have already proven they can bypass it with ease.