Greek authorities warn that it is hard to detect Islamic State militants crossing into Europe or the United States. In an interview with French AFP news agency, Moria police captain of the main registration camp on Lesbos, Greece, said that “if (the refugees) are not already registered in the database, it’s nearly impossible.”

Once registered, the refugees have photos taken, are fingerprinted and get quizzed by European authorities. Despite the detailed security checks, jihadists have been able to pass through undetected.

At least two of the suspects in the Paris attacks on November 13 passed through Greece posing as Syrian refugees fleeing war. Ahmad al-Mohammad was identified as a terrorist only after he took part in the multiple attacks in Paris. Before that, he was treated as a refugee. The mastermind of the attacks, Belgian national Abdelhamid Abaaoud, reportedly came into Europe through Greece and then traveled back to Syria undetected, though this has yet to be confirmed by Greek authorities as no source has provided tangible evidence regarding this claim.

This proves that the challenge of detecting terrorists is unparalleled.