Western intelligence services examined Thursday a potential treasure trove of leaked IS registration documents, some of which were deemed likely to be authentic by the German intelligence service despite the doubts of many experts.
The information, which could help track down current and former Islamic State group jihadists, was included in forms which new recruits had to fill out, British and German media reported.
The documents contain details like names, dates of birth and phone numbers for people from 51 countries including from Britain, northern Europe, the Middle East, north Africa, the United States and Canada.
British broadcaster Sky News said it had shared with British security services documents with the identities of some 22,000 IS group members.
While Britain\’s interior ministry said it would not comment on national security matters in line with normal policy, a spokeswoman for Prime Minister David Cameron said the government would "look at how this information can be used in the fight against Daesh," another term for Islamic State.
"If it can, then we would welcome that," she said, adding that the government had not been aware of the story before it was reported by Sky.
German federal police spokesman Markus Koths said there was "a very high probability" that documents reported by German media as identifying German jihadists were genuine.
"We are therefore taking them into account as part of our law enforcement and security measures," he added.
Fredrik Milder, a spokesman for Sweden\’s Sapo intelligence service, meanwhile told TT news agency: "We are aware of the existence of this list."
"Our security services have received such information," he said.
The developments follow German media reports earlier this week about a questionnaire which new IS recruits from Germany had to fill in.
German Interior Minister Thomas de Maiziere said they would help "a better understanding of the structures" of IS and pave the way for "faster, clearer investigations and higher prison sentences".
Experts said the documents would be valuable if authentic, although they pointed out mistakes and uncharacteristic language in the forms.
Richard Barrett, formerly a senior figure in Britain\’s MI6 overseas intelligence service, said the leak would be "an absolute gold mine of information of enormous significance and interest".
But Charlie Winter, a researcher at Georgia State University, said: "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."
The Arabic name for "The Islamic State in Iraq and Syria," IS\’s previous nomenclature, is written in two different ways, including one that is not consistent with past practice.
Files documenting the deaths of IS militants use the words "date of killing" instead of the typical jihadist term "martyrdom."
Romain Caillet, an independent jihadism expert, also noted that some documents feature a second, circular logo not previously used on IS files.
Syrian opposition news website Zaman al-Wasl said there were thousands of repetitions in the leaked documents and the names of only 1,700 people could be identified in the 22,000 documents.
Sky reported that a disillusioned former member had handed over the documents on a memory stick that had been stolen from the head of the group\’s internal security police.
Questions on the documents include asking recruits their blood type, mother\’s maiden name, "level of sharia understanding" and previous experience, it said.
Names on the list include Reyaad Khan, a 21-year-old British jihadist who Britain\’s government said was planning attacks on Britain before being killed by a British drone in Syria last year.
Another on the list was Junaid Hussain, a British computer hacker described by British authorities as a key IS operative, who died in a US air strike last year.
The alleged leak comes with Western security services on high alert against the possibility of fresh jihadist massacre following the Paris attacks spree last November.
On Monday, Britain\’s most senior counter-terrorism police officer warned of the risk of "spectacular" attacks targeting the "Western lifestyle".
"You see a terrorist group which has big ambitions for enormous and spectacular attacks, not just the types that we\’ve seen foiled to date," said Mark Rowley, assistant commissioner at London\’s Metropolitan Police.
British police say they foiled seven plots between late 2014 and early 2016.