The disclosure is a potential boon to Western intelligence, but also illustrates discontent in the ranks of the so-called "Islamic State." In total, broadcaster Sky News said it had received more than 22,000 documents.
A disaffected "Islamic State" (IS) fighter has given a memory stick containing tens of thousands of files to British broadcaster Sky News, many of them identifying IS fighters and even their family members.
Sky News claims that the portable computer drive was stolen from the group's internal security police chief.
The information would seem to suggest IS has a penchant for paperwork. The electronic drive includes forms that would-be jihadis had to fill out in order to be accepted into the terror organization.
The 23-point questionnaire includes queries about blood type, mother's maiden name, contact telephone number and previous experience. It also asks about applicants' understanding of Sharia law.
Chris Phillips, managing director of counter terrorism consultancy International Protect and Prepare Security Office, told news agency AFP that the disclosure is a potential game-changer in the West's war against the IS.
"This could be a massive development," Phillips said. "It shows how ISIS is vulnerable to its own people turning against them," he said, using an alternative acronym for the terror group. "The potential for security services identifying unknown terrorists is greatly enhanced."
German authorities said earlier this week that they had obtained information on Islamic State fighters based in Germany, with Interior Minister Thomas de Maiziere saying he was confident of their authenticity, and their usefulness to law enforcement. Based on the similarities in the type of information obtained, such as the IS application questions, the data appeared to stem from the same source.
A boon for Western intelligence
The documents could be used to hunt down current fighters, diminish the flow of would-be jihadists from Europe and the United States, and perhaps compel others to turn against the group.
"Understanding how people have traveled and who recruited them, is a key opportunity to reduce those leaving in the future," Phillips told AFP.
The British broadcaster said they obtained the documents from a man in Turkey who goes by the name of Abu Hamed, a former Free Syrian Army member who joined IS. They say that he stole the memory stick of documents and that he left IS because Islamic rules had collapsed inside the group.
Hamed said IS had given up on its headquarters in the Syrian city of Raqqa and was moving into the desert, adding that former soldiers from the Iraqi Baath party of executed dictator Saddam Hussein had taken over the organization.
This isn't the first time IS documents have been leaked. Like previous leaks, the latest tranche of documents show the militants to be highly bureaucratic, and obsessed with rules governing minute details of everyday life.
If verified, the cache of members' identities would be the most significant leak so far relating to the group.