More than a dozen Britons and a handful of Americans are among Islamic State fighters reportedly named in a cache of 22,000 documents obtained by German intelligence.

Britons identified in the documents so far had previously been revealed to the public and are dead, killed in US-led strikes, or their whereabouts unknown. Sixteen Britons are thought to be on the list, among them Junaid Hussain and Reyaad Khan.

The documents, thought to be from a border crossing into Syria, are questionnaires of each would-be recruit. There are 23 questions, including names, date and place of birth, hometown, telephone number, education and blood type.

Germany’s interior minister, Thomas de Maizière, confirmed the documents were real and they would facilitate “speedier, clearer investigations and stricter prison sentences” for those returning from Syria and Iraq. De Maizière said the materials help clarify “the underlying structures of this terrorist organisation”.

A spokesperson for the BKA, the German federal police, confirmed that the agency was in possession of the cache of documents, adding that experts determined their authenticity. German officials did not specify how the agency had got the documents, nor how many names had been found within them.

German media reported that the questionnaire asked would-be Isis recruits about any previous experience they had in jihad and whether they were prepared to be suicide bombers.

The existence of the documents was revealed by the Munich-based Süddeutsche Zeitung paper and German broadcasters WDR and NDR on Monday evening. Zaman al-Wasl, a pro-opposition Syrian news website, published examples of the questionnaires on Tuesday.

Sky News claimed on Tuesday that it too has obtained copies of what appeared to be the same documents, containing about 22,000 names. It said the they were passed on a memory stick stolen from Isis internal security police by a former Free Syrian Army convert who later became disillusioned with Isis.

Sky News claimed it has obtained copies of the same ISIS documents Photograph: Sky News

The documents held by the German authorities seem to have been collected at the end of 2013. Even the lowest estimate of the numbers that crossed the border in that period indicates the sheer scale of volunteers to Isis. The documents will be useful to intelligence agencies in confirming names and details of people suspected of joining Isis. However, it was reported that there are names not previously known to the intelligence services.

Zaman al-Wasl reported that personal details of 1,736 fighters from 40 countries had been revealed – a quarter were Saudis and the rest predominantly Tunisian, Moroccan and Egyptian.

The documents, written in Arabic and stamped with logos used by Isis, allegedly contain details of 16 British fighters, four from the US and six from Canada, as well as recruits from France and Germany.

Intelligence agencies have estimated that about 700 Britons have joined Isis.