Buried in the midst of thousands of Islamic State files passed to Sky News we discovered a spreadsheet different to the rest of the documents.

The names of Islamic fighters, their pseudonyms, their countries of origin and contact numbers for family members, we had seen before.

What marked this file out was its title: The Martyrs.

Previously unheard of, this was a totally secret brigade. A brigade made up of men who had joined Islamic State to die as suicide bombers.

The files revealed the names of 123.

They came from a variety of countries: France, German, Spain, Tunisia and Egypt.

What is interesting in the files is the number of times that Belgium or Belgian cities are mentioned.

We can reveal that 25 Belgians are identified.

There are 48 references to Belgian nationals within the registration papers.

There are 70 references to the country which include their sponsors who guaranteed their entry to the terror group, family members and telephone numbers.

Islamic State, like many previous jihadi groups, has used suicide attackers to overrun their enemies' positions from Libya to Pakistan and Afghanistan. All in traditional war zones. What marks this brigade out as different is that it appears to have been made up, in the large part, by killers trained to carry out attacks in the peaceful cities of Europe and beyond.

Death squads sent out to attack away from Syria and Iraq, away from the battlefield.

The files we have published over the past week or so list all the fighters' intended specialisms.

Fighter, infiltrator and Martyr were standard pieces of information requested. All these men ticked the Martyr box. In translation it is suicide attacker.

The registration form of Mohammed Belkaid, first reported by Sky News from our files after he opened fire on police in Brussels last week and was killed, showed that he too was part of the Martyrs' Brigade.

In Syria he is likely to have joined one of the training camps and the Islamic State training programme in their stronghold of Raqqa.

Sky News has previously revealed the existence of these foreigners' camps, which train fighters to carry out attacks outside Syria and Iraq.

Counter Terrorism expert Professor Andrew Silke said ISIS seeks out recruits for its martyrs' brigade that have a series of qualification.

He said: "One of the things that the movements are interested in is 'have we got a candidate who is willing and able to carry out a suicide attack? Because there's a value in that.

"Another issue ... is 'could this person operate in the West? Have they got the language skills? Do they fit in with the culture? Do they come from that particular region, because if they do, their ability to go back and operate (there) ... is much greater than sending somebody from the Middle East."

Some of the Belgians we can easily identify.

Redwana Mohammed Hajaoui also known as Abu Khalid al Maghribi, crossed into Syria in February 2014. He later appeared in an Islamic State propaganda video.

Mesut Cankarturan also known as Abu Abdullah al Beljiki from Bruges, crossed into Syria in March 2014. He later died near Deir ez-Zor.

During our investigations Sky News has learned from former ISIS members that the recruits were trained not just to carry out attacks but to be trainers as well, raising the specter of further developing terror cells.

The analysis of these files will take a long time; certainly the security services are gearing up for a long fight against Islamic State and its terror gangs.

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