Nationals from at least 51 countries had to give up their most personal information as they joined the terror organisation. Only when the 23 question form was filled in were they inducted into ISIS.
A lot of the names and their new so-called “Islamic State”( also known as ISIS , ISIL , IS and Daesh) names on the registration forms are well known.
Adel Abdul Bary was initially suspected by MI5 of being Jihadi John before being ruled out
For example Abdel Bary, a 26-year-old from London joined in 2013 after visiting Libya, Egypt and Turkey. He is designated as a fighter but is better known in the UK as a rap artist. His whereabouts are unknown.
Another terrorist named in the documents, now dead after being targeted in a drone strike, is Junaid Hussain, the head of Islamic State media wing who along with his wife former punk Sally Jones, plotted attacks in the UK. Her whereabouts are unknown.
Reyaad Khan, 21, from Cardiff, was killed in an RAF drone strike last year
Reyaad Khan from Cardiff, who also entered in 2013, is also among those found among the registration forms. He was well known for appearing in a highly produced Islamic State propaganda video. He was later killed.
"But the key breakthrough from the documents is the revealing of the identities of a number of previously unknown jihadis in the UK, across northern Europe, much of the Middle East and North Africa, as well as in the United States and Canada."
Their whereabouts are crucial to breaking the organisation and preventing further terror attacks.
Many of the men passed through a series of jihadi hotspots - such as Yemen, Sudan, Tunisia, Libya, Pakistan and Afghanistan - on multiple occasions, but were apparently unchecked, unmonitored and able to both enter Syria to fight and then to return home.
One of the files marked "Martyrs" detailed a brigade manned entirely by fighters who wanted to carry out suicide attacks and were trained to do so.
The man who stole it was a former Free Syrian Army convert to Islamic State who calls himself Abu Hamed.
Disillusioned with the Islamic State leadership, he says it has now been taken over by former soldiers from the Iraqi Baath party of Saddam Hussein.
He claims the Islamic rules he believed have totally collapsed inside the organisation, prompting him to quit.
I met him in a secret location in Turkey, and he said ISIS was giving up on its headquarters in Raqqa and moving into the central deserts of Syria and ultimately Iraq, the group's birthplace.
Asked if the Daesh leaked files could bring the network down he nodded and said simply: "God willing".
From the attacks in Tunisia and the Bataclan massacre in Paris it is clear that ISis is refocusing its base of operations abroad and is intent on carrying out high profile attacks in Western countries, something that security chiefs across Europe are warning about right now.
According to the report, Sky News has informed the authorities about the haul.
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