The Islamic State trains its fighters to pass refugee vetting procedures and come across as ordinary asylum seekers, according to a new report from German’s Federal Intelligence Service (BND).

ISIS has a handbook on how a person can infiltrate among regular refugees and not attract attention from law enforcement agencies. Militants know what answers to give in interviews with police and in conversations with ordinary refugees, intelligence sources told German newspaper Die Welt.

All nine terrorists behind the attack in Paris exactly one year ago travelled to Europe from Syria disguised as refugees and passed security without drawing suspicion. BND warns that more trained terrorists may attempt to take the same route and carry out attacks.

Belgian foreign minister Jan Jambon suggested Sunday that as many as 5,000 European ISIS fighters may look to return to Europe as the terrorist group is under heavy pressure in strongholds Raqqa and Mosul.

“Belgian fighters are helping defend the strongholds, but we must be aware that there are not only 200 of our countrymen there, but between 3,000 and 5,000 Europeans,” Jambon told state broadcaster RTBF. “All the intelligence agencies are trying to monitor the situation and exchange information. If the wave comes, we must be ready.”

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