German federal police spokesman Markus Koths said there was “a very high probability” that documents reported by German media as identifying German jihadists were genuine.
“The Federal Criminal Office has knowledge of this type of document of the so-called Islamic State,” said police spokesman Markus Koths, referring to Isis registration forms reported on by Süddeutsche Zeitung and public broadcasters NDR and WDR.
“We assume that there is a very high probability that they are genuine documents,” Koths said in a short statement. “We are therefore taking them into account as part of our law enforcement and security measures.”
Koths declined to give details about how many documents his office has access to or their contents, and stressed that he was not commenting on the authenticity of specific documents obtained by German or other media.
The information, which could help track down current and former Isis jihadists, was included in forms which new recruits had to fill out, German and British media reported.
British broadcaster Sky News said it had shared with British security services documents with the identities of some 22,000 Isis group members.
While Britain's interior ministry said it would not comment on national security matters in line with normal policy, a spokeswoman for Prime Minister David Cameron said the government would “look at how this information can be used in the fight against Daesh,” another term for Isis.
“If it can, then we would welcome that,” she said, adding that the government had not been aware of the story before it was reported by Sky.
The developments follow German media reports earlier this week about a questionnaire which new Isis recruits from Germany had to fill in.
German Interior Minister Thomas de Maiziere said they would help “a better understanding of the structures” of Isis and pave the way for “faster, clearer investigations and higher prison sentences”.
Experts said the documents would be valuable if authentic, although they pointed out mistakes and uncharacteristic language in the forms.
Richard Barrett, formerly a senior figure in Britain's MI6 overseas intelligence service, said the leak would be “an absolute gold mine of information of enormous significance and interest”.
But Charlie Winter, a researcher at Georgia State University, said: “There would be big alarm bells for me, because when I've seen inconsistencies like that in the past they've been on really shoddily-made forgeries.”
The Arabic name for “The Islamic State in Iraq and Syria,” Isis's previous nomenclature, is written in two different ways, including one that is not consistent with past practice.
Files documenting the deaths of Isis militants use the words “date of killing” instead of the typical jihadist term “martyrdom.”
Romain Caillet, an independent jihadism expert, also noted that some documents feature a second, circular logo not previously used on Isis files.
Syrian opposition news website Zaman al-Wasl said there were thousands of repetitions in the leaked documents and the names of only 1,700 people could be identified in the 22,000 documents.
Detailed list of questions
Sky reported that a disillusioned former member had handed over the documents on a memory stick that had been stolen from the head of the group's internal security police.
Questions on the documents include asking recruits their blood type, mother's maiden name, “level of sharia understanding” and previous experience, it said.
The alleged leak comes with Western security services on high alert against the possibility of fresh jihadist massacre following the Paris attacks spree last November.