Tens of thousands of documents, containing names, addresses, telephone numbers and family contacts of Islamic State jihadis, have been obtained by Sky News.
Nationals from at least 51 countries, including the UK, 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 IS.
A lot of the names and their new Islamic State names on the registration forms are well-known.
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 jihadi named in the documents, now dead after being targeted in a drone strike, is Junaid Hussain, the head of Islamic State's media wing who along with his wife former punk Sally Jones, plotted attacks in the UK.
Her whereabouts are unknown.
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.