Young girls place flowers in front of the Obelisk of Theodosius, the scene of a suicide bomb attack, at Sultanahmet Square in Istanbul, Jan. 13, 2016. Turkish authorities detained three Russian nationals suspected of links with the Islamic State following the attack that killed 10 tourists. Reuters

They're called "The Martyrs." Among the brutal ranks of the Islamic State group, they are the ones who are ready to die as suicide bombers.

The militant group also known as ISIS counts at least 123 members among its secretive suicide brigade, Sky News reported Tuesday. The would-be suicide bombers come from France, Germany, Spain, Tunisia and Egypt. About 25 are Belgian nationals.

The report came as suicide bombers spread terror in Brussels Tuesday, killing and wounding dozens of victims at the airport and a metro station. ISIS has claimed responsibility for the attack.

ISIS has employed suicide bombers to overrun their enemies' positions in war zones, but many of the "martyrs" have been trained to execute attacks in Europe and other Western nations. One member of the Martyrs' Brigade was Mohammed Belkaid, who was killed after he opened fire on police in Brussels last week and is believed to have joined one of ISIS' training camps in the group's stronghold of Raqqa, Syria.

Andrew Silke, a counterterrorism expert, told Sky News ISIS looks for potential suicide bombers who are willing and able to carry out the task. "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," he said.

The existence of the suicide brigade was discovered on a spreadsheet found among other ISIS documents and studied by Sky News. The file was called "The Martyrs." Sky News obtained the documents from a memory stick stolen by an informant from the head of ISIS’ internal security police. It includes information about 22,000 militants from 51 countries, including their names, nationalities, addresses, phone numbers, family contacts and blood types.