It almost sounds too good to be true: a huge list of Islamic State fighters, leaked from inside the organisation's own internal security division, potentially representing a treasure trove of detailed information on the group to be picked apart by intelligence agencies.

This week, Sky News revealed it had acquired this list, reportedly registration documents for Islamic State fighters from 2013.

There were tens of thousands of names on the list from at least 51 countries, the British news organisation reported, making it perhaps the most important Islamic State leak since the Sinjar records of al-Qaida in Iraq - documents discovered by US forces in Iraq that contained the details of over 600 fighters from that group, a precursor to the Islamic State.

Sky News reported the documents were contained on a memory stick that was stolen from the head of Islamic State's internal security police.

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Though it was the first Western outlet to publish details of the list, it appears that an opposition Syrian outlet, Zaman al-Wasl, had been able to publish details of the list first, shortly before Sky News published their story.

German authorities are also believed to have a list that is either similar or the same, according to reports in the German press from earlier this week.

The scale and detail of the information certainly seems dramatic. In registration questionnaires, foreign fighters had been asked to give up not only their own details, but also information about who had smuggled them into Islamic State-held territory and their links back home.

They were even asked if they'd prefer to fight or die in a suicide bombing.

Former global terrorism operations director at MI6 Richard Barrett told Sky News the list was "an absolute goldmine of information of enormous significance and interest to very many people, particularly the security and intelligence services."

US officials said that they were aware of the reported cache but have reached no conclusions about the authenticity of the documents. The CIA declined to comment. One US official, however, did say that the records would be "fantastic if true."

Whether they actually are true or not remains to be seen. A number of stylistic details from the list have stood out as inconsistent to experts, including the fact that the Arabic version of the group's previous name, "The Islamic State in Iraq and Syria," is written two different ways, a previously unseen circular logo appears on the documents, and the column used to refer to the death of the fighter did not use the jihadist term for "martyrdom," as seen widely in other Islamic State documents.

Additionally, while Sky News suggests there are tens of thousands of names on the list, Zaman al-Wasl reports that their own list features 22,000 entries. But, due to repetitions, only 1,700 people are actually identified.

Charlie Winter, a terrorism analyst currently at Georgia State University, told Agence France Presse that these unusual details gave him reason to pause.

"There would be big alarm bells for me, because when I've seen inconsistencies like that in the past they've been on really shoddily made forgeries," Winter said.

Aymenn Jawad al-Tamimi, another analyst who has written guides to recognizing faked Islamic State documents, said that he remains unconvinced that the documents are "wholly authentic," noting that a financial motivation could have lead to the documents being faked or authentic data being tampered with somehow.

Others have suggested the document's obvious flaws suggested they were indeed the work of an overworked bureaucracy, rather than a forger.

"If these are forgeries, why aren't they more artfully done?" William McCants, Director of the Brookings Institute's Project on US Relations with the Islamic World, said in an email. Peter Neumann, a jihadism researcher with London's King's College, suggested on Twitter that faking this many documents was simply unrealistic.

Rita Katz, co-founder of the Search for International Terrorist Entities Intelligence Group, said that a number of details can be cross-referenced with other reports, suggesting that the information contained in the document is real. While she acknowledged the stylistic oddities within the document, she suggested this is indicative of their bureaucratic rather than theological or propagandistic function.

"ISIS, just like Al Qaida, is a very detailed organisation," Katz explained, using an acronym to describe the Islamic State. "These forms are for administrative purposes - it's really more of a technical thing."