Sky News claims to have taken possession of tens of thousands of the terror group's registration forms, "containing 22,000 names, addresses, telephone numbers and family contacts of Islamic State jihadis."
The news organization got the files on a USB stick that was stolen from the head of ISIS's security police by a member who became disillusioned, Sky News said.
Get Data Sheet, Fortune’s technology newsletter.
The leak includes the details of many known ISIS fighters, but also many people who were not previously known to have joined the group from Europe, North America, the Middle East and elsewhere.
Some of these people may have returned home after going through ISIS's bureaucracy, making the documents — if real — of intense interest to authorities around the world. Sky News said it has informed the authorities of what it has.
The article theorizes that many of the phone numbers in the documents will be currently used by the ISIS members themselves, which will prove extremely useful to agencies that can monitor such things. One of the files in the cache apparently identifies recruits who are willing to martyr themselves.