German authorities and UK's intelligence experts have said the details of 22,000 Islamic State supporters in a memory stick passed on to British media by a disillusioned former member of terrorist gourp appear to be genuine.
Germany's interior minister Thomas de Maiziere said the files containing information on Islamic State recruits can be assumed to be genuine.
"The German Federal Bureau of Investigation acts on the assumption that the documents are authentic," de Maiziere said, BBC reported.
"We can also improve our understanding of the structures of this terror organisation," he added. "And possibly, it will discourage young, radicalised people, who believe they are doing something good if they become a member of a criminal organisation."
A former member of Isis, calling himself Abu Hamed, passed on the files to a journalist with Britain's Sky News. He said he had stolen the files from the head of the terrorist group's internal security force.
While British Home secretary Theresa May said she could not comment on "specific national security matters", a former head of MI6 said the files contained a gold mine of information.
"It will be an absolute goldmine of information of enormous significance and interest to very many people, particularly the security and intelligence services," Barrett told Sky News.
The files were earlier published by by Zaman Al-Wasl, a Qatari-based Syrian opposition news website.
The website had said the files contained the names and other details of 22,000 Isis supporters, but many of them could be duplicates.
Germany's Sueddeutsche Zeitung newspaper also said it received the files from a "trusted source".
The files published by Zaman Al-Wasl contain information that could be cross-verified with official national databases on Isis supporters.
Two Isis supporters on trial n Germany, Kerim Marc B and Abdelkarim B, are on the list, while other two Germans on the list have appeared in IS videos. The list also names 16 Britons who are known to have fought for IS and a Dutch teenager who was killed in a US air strike on a Daesh stronghold in Syria.