A former fighter with the self-proclaimed Islamic State, or IS, has reportedly leaked thousands of files from the militant group.
The discovery could help global intelligence authorities expose supporters of the organisation from more than 50 countries.
Documents belonging to IS have been handed over to a British news outlet, potentially revealing the names of more than 20,000 supporters.
A memory stick containing the files was passed from an apparent former member of the Free Syrian Army, who later joined IS, to a journalist with Sky News.
The man, calling himself Abu Hamed, says he became disillusioned with the organisation and stole the records from its internal security force.
(Translated)"I want to say to people on the inside of the organisation, 'The organisation is a lie. It is not Islam. There is nothing that ever follows the sharia or Islam. It's far from Islam.'"
He alleges the group has been taken over by ex-soldiers from Iraq's Baath party, formerly led by then-president Saddam Hussein.
If genuine, the documents could provide global intelligence authorities with important information in the fight against IS, helping them identify recruiting agents and foreign fighters.
Most of the members are believed to originate from countries in the Middle East, while European numbers are dominated by recruits from France, Germany and Britain.
An estimated 150 Australians are also believed to be fighting in overseas groups such as IS, also known as ISIS or Daesh.
German Interior Minister Thomas de Maiziere says the leak could have a huge effect on national and international operations.
(Translated)"They offer the great opportunity to find evidence that German citizens participated in terror activities of the so-called Islamic State. In this way, we could speed up and enhance our investigations, and there will be tougher prison sentences. We can also improve our understanding of the structures of this terror organisation."
Some documents reportedly among the thousands included are enrolment forms for IS supporters and their relatives, as well as a list of prospective suicide bombers in a file marked "Martyrs."
The former head of global counter-terrorism at Britain's MI6 intelligence service, Richard Barrett, has described the leak as "an absolute goldmine of information."
The German newspaper Suddeutsche Zeitung claims it was the first outlet to obtain the documents.
Its foreign editor, Stefan Cornelius, has told the BBC they offer an unprecedented look at the inner workings of the organisation.
"Those documents are from the Turkish-Syrian border. They are offered there for money. It gives some proof on the state of ISIS right now, since many of those members and those close to the terrorist group are trying to make money, quite honestly. Because, obviously, ISIS is in a desperate financial state. So, you are offered documents, you are offered videos, you are offered artefacts from the ISIS, and you can, basically, buy this."
A former member of the British government's counter-extremism taskforce, Haras Rafiq, has told Sky News he believes the disclosure is only the beginning.
He points to what he believes are deeper divisions within IS.
"For me, what is really, really important is that there has been somebody within the organisation, within the area of land that ISIS occupies, that has been prepared to defect, that has been prepared to take his or her life into their own hands and actually leak information that they consider to be vital to the aims and objectives of the organisation. And this shows really, for me, that there's a fracture within ISIS, within Daesh, the so-called Islamic State, and we're going to see more and more defectors, I think."
