According to German media reports, the names of three of the Paris attackers appear in files leaked from the ISIS.
The three are believed to have carried out the worst attack, at the Bataclan concert hall, where 90 people died in November 2015.
The ISIS files, obtained by German, UK and Syrian opposition media, are said to identify thousands of jihadist recruits from at least 40 countries.
German officials said the files could be assumed to be genuine.
Roughly 22,000 fighters are reportedly identified by the documents, with one file for each recruit listing a name, address and other information. However, many of the names given may be duplicates.
Among them are Samy Amimour, Foued Mohamed-Aggad and Omar Ismail Mostefai, the three men who attacked the Bataclan during Eagles of Death Metal’s concert, killing 90 people.
WDR said the files indicate that the three men entered ISIS territory in 2013 and 2014.
Germany’s Interior Minister Thomas de Maiziere earlier said the information contained in the files could help to prosecute ISIS fighters, and help prevent future recruitment.
Files were first published online (in Arabic) by Zaman Al-Wasl, a Qatari-based Syrian news website.
Dutch media identified Abu Jihad al-Hollandi from the documents as Amsterdam teenager Achraf Bouamran, who was killed in a US air strike on the Syrian ISIS stronghold of Raqqa in January 2015. His file reads: “Born 1997. Moroccan origin. Wants to be a fighter.”
Counter-terrorism police in Germany are studying the documents.
“The German Federal Bureau of Investigation acts on the assumption that the documents are authentic,” Thomas de Maiziere said.
Sky News said the documents came from a man called Abu Hamed, an ISIS fighter who said he had become disillusioned with the group’s leadership and stolen a memory stick from the head of the ISIS internal security force before handing it over in Turkey.