German counter-terrorism police are investigating documents said to reveal the identities of a large number of fighters of so-called Islamic State.
The files, obtained by German and British media outlets, are said to be recruitment questionnaires that identify thousands of supporters of the Islamic State group from 50 countries.
They appear to contain names, addresses and phone numbers.
German officials say they are working on the belief the files are genuine.
A ministry statement said the papers offered “a great chance to identify Germans taking part in terrorist activities of the so-called IS”.
It added: “It allows us to understand the structure of this terror organisation and perhaps it will serve as a deterrent for young radicalised people, who think they are doing something good, but who now may realise they are the victims of a criminal organisation.”
UK Home Secretary Theresa May said she could not comment on “specific national security matters”, but added: “Of course, I have seen the reports that have taken place.”
She said the IS group “poses a severe threat… it is important for us to work together to counter this threat”.
Sky News said the files contained 22,000 names, addresses, telephone numbers and family contacts of jihadists. It said the documents were obtained from a man called Abu Hamed, an Islamic State fighter who became disillusioned with the group’s leadership.
He says he stole a memory stick containing the information from the head of Islamic State’s internal security force and handed it over in Turkey.
The questionnaire has 23 sections, that also includes countries travelled through to reach Syria, fighting experience and a line for the “date and place of death”.
Zaman Al-Wasl, a Syrian independent news website, said in January it had also obtained documents that “included 23 fields” and listed the “real names of IS fighters and their jihadi backgrounds, nationalities and hometown addresses”.
However, it says that only 1,700 names are identifiable.
Examining samples from this site, the BBC found that the IS group had recruited jihadists from more than 40 countries, 72% of them Arabs. The main sources were Saudi Arabia, which supplied 25%, followed by Tunisia, Morocco and Egypt. Of European nations, Turkey and France were the leading suppliers.
One document found from the site relates to a Briton called Abu Jibril al-Britani. The questionnaire says he was born of Bangladeshi origins in 1995, entered Syria on 31 August 2013, and was offering four areas of work – “fighter, sharia related, security work and admin”.
German media also reported this week they had documents.
The Sueddeutsche Zeitung newspaper said it had obtained “dozens” of similar documents on the Turkey-Syria border.
German media have pulled out a number of names, including Kerim Marc B and Abdelkarim B. who they say are currently on trial separately in Germany.
Some of the papers are stamped Islamic State in Iraq and the Levant, which pre-dates the rebranding to Islamic State in June 2014.
Analysts say the IS group is known to be bureaucratic, so such lists may not be surprising.
Shashank Joshi, a senior research fellow at the Royal United Services Institute think-tank, told the Press Association the papers could be “incredibly important”.
He said: “It is a law enforcement gold mine. It means it might make it easier to prosecute those who have returned.”
IS, a fundamentalist Sunni Muslim group also known as Daesh, is notorious for its brutal methods in gaining territory in Iraq and Syria.