The Federal Criminal Police Agency (BKA) acquired thousands of files produced by the "Islamic State" (IS) militant group comprising personal information on foreign fighters, including those from Germany, the Munich-based newspaper "Süddeutsche Zeitung" reported on Monday.

The material is based on "entrance interviews" given to foreign fighters when they first arrive in 'IS'-held areas.

The report said that those who freely volunteer themselves to join the militant group must first answer 23 questions, which include information on their previous residence, religion, family members, education and "so-called jihadist experience."

Foreigners, namely those from outside Iraq and Syria, must also specify if they wish to be used as a fighter or suicide bomber.

"Süddeutsche Zeitung" added that the German attorney general knew of the files, noting that they were being used by authorities to identify citizens returning to Germany after having fought with the militant group.

"We believe that it is very likely that these are real documents," the BKA told "Süddeutsche Zeitung," without providing further details on the files.

German public broadcasters WDR and NDR also contributed to the investigation.

In August 2015, the militant group released a video calling for more German-speaking sympathizers to join the 'IS' in Syria and Iraq. Two apparent members of the group threatened attacks on German soil.

According to domestic intelligence agencies, more than 600 German citizens have joined the 'IS.'

ls/jil (SZ, WDR, NDR)