Thousands of documents identifying 22,000 supporters of ISIS in over 50 countries were handed over to Sky News by a disillusioned former member of the group, the British television channel reported on Thursday.

Thousands of files have surfaced with personal data on members of ISIS – documents that might help authorities track down and prosecute foreign fighters who returned home after joining the extremists, or identify those who recruited them in the first place.

Germany’s federal criminal police said Thursday they are in possession of the files and believe they are authentic.

The announcement came after Britain’s Sky News reported it had obtained 22,000 ISIS files that detail the real names of fighters for the group, where they were from, their telephone numbers and even names of those who sponsored and recruited them. In a joint report, Germany’s Sueddeutsche Zeitung newspaper in Munich and broadcasters WDR and NDR reported independently Monday they had obtained “many dozens” of pages of such documents itself.

“This is a huge data base – there are more than something like 22,000 names, so this is very, very important,” said Dalia Ghanem-Yazbeck, a research analyst at the Carnegie Middle East Center.

She said the files would “definitely” help international security services, including those in Arab countries, to confirm the identities of those who have already left to fight for ISIS, to discover the identities of new fighters, and to help them in identifying those who return home from Syria and Iraq.

Sky said the files, obtained at the border between Turkey and Syria, were passed to them on a memory stick stolen from the head of the extremist group’s internal security police by a former fighter who had grown disillusioned with the group.

Sueddeutsche Zeitung and the German broadcasters reported they also had obtained the files on the Turkey-Syria border, where they said ISIS files and videos were widely available from anti-ISIS Kurdish fighters and members of ISIS itself.

The documents highlight the bureaucratic work of the highly secretive extremist group that has spread fear through its brutal killings and deadly attacks in its self-declared caliphate of Syria and Iraq, as well as in places like France, Turkey, Lebanon, Yemen and Libya.

The information could help the US-led coalition that is fighting the militant group by aiding in a crackdown on the extremists’ foreign fighter networks, said US Army Col. Steve Warren, a spokesman for the coalition.

He said that while he was not able to verify the documents, he hoped that “if there is a media outlet that has these names and numbers, I hope they publish them.”

That would help bring attention to the problem of foreign fighters joining ISIS and also would help authorities to crack down on the problem, he said.

“This would allow the law enforcement apparatus across the world to become much more engaged and begin to help do what we can to stem this flow of foreign fighters – so we’re hopeful that its accurate and if so we certainly plan to do everything we can to help,” he said.

Both Sky and Sueddeutsche Zeitung reported the documents were forms with 23 questions to be filled out by recruits when they were inducted into ISIS. Sky said they included nationals from at least 51 countries, including the US and Britain.

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