The names, birth dates and phone number of some 22,000 members of the extremist group were contained in the documents leaked to the German newspaper, Süddeutsche Zeitung, and the British media.
The Italian citizens were Rawaha al Itali, a rapper of Moroccan origin who went by the name of Anas El Abboubi, and Abu Ishaq al-Tunisi, a 39-year-old of Tunisian origin, Corriere reported.
The 21-year-old rapper, who had both an Italian and Moroccan identity card, travelled to Syria from Brescia in early 2013 and was killed there that summer. Al-Tunisi had lived in five EU countries, including Italy, France, Belgium, Holland and Germany, over a four year period, Corriere said.
German federal police said on Thursday that the documents are very likely to be genuine, although analysts have cast doubt.
The information, which could help track down current and former Isis jihadists, was included in forms which new recruits had to fill out, German and British media reported.
The information included 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.
The developments follow German media reports earlier this week about a questionnaire which new Isis recruits from Germany had to fill in.
German Interior Minister Thomas de Maiziere said they would help provide “a better understanding of the structures” of Isis 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,” Isis's previous nomenclature, is written in two different ways, including one that is not consistent with past practice.
Files documenting the deaths of Isis 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 Isis 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.
Detailed list of questions
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.
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.