The German research team said it had now obtained data on some 2,000 IS members from as many as 40 countries.
Some papers reportedly also made indirect reference to Abdelhamid Abaaoud, the organiser of the Paris killings who recruited old friends and other small-time delinquents to help him carry out the attacks in the French capital which killed 130 people and wounded hundreds of others.
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Germany's federal police said Thursday they were taking the cache of documents seriously but proceeding with caution as some experts have pointed out discrepencies in the language and logos used in the forms.
French Interior Minister Bernard Cazeneuve echoed this need for caution.
"We are extremely interested in information which would allow us to ... neutralise terrorists but such information has to be authenticated," he said in Washington.
Britain's Sky Media 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.
If the documents are proved to be genuine, they could help intelligence services around the world track down people who have travelled to countries such as Syria and Iraq to join IS.
To read our coverage of the January 2015 Charlie Hebdo killings click here