We investigate the ISIL cell responsible for the attacks in Paris and Brussels.

Correction, 26/01/2017: : An earlier version of this article stated that the Paris attacks had the highest death toll of an attack in Europe since 1988. This was incorrect. The Madrid train bombing in 2004 claimed the lives of 190 people.

On November 14, 2015, a state of emergency was declared across France, after 130 people were killed and 368 injured in a series of coordinated attacks over a nearly four-hour period, beginning outside the national stadium during a football match and followed by several shootings and a suicide bombing at cafes and restaurants.

The police investigation into it focused on fighters belonging to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

Questions were raised about the extent of ISIL's presence on the continent.

Four months later, on March 22, 2016, two bombs were detonated at Brussels airport and one bomb at a metro station in the heart of the Belgian capital. The attacks were claimed by ISIL. Forty-eight people were killed, and more than 300 were injured.

Following the Paris attacks, Brussels was put on the highest level of security alert. But it also became the focal point of the search for the cell, considered the most dangerous and wanted group in Europe.

Four Belgian nationals became the prime suspects. This ISIL cell, headed by Abdelhamid Abaaoud, was based in Molenbeek, an ethnically diverse district of Brussels. The city emerged as a recruiting ground for ISIL and a base for many of its activities in Europe.

ISIL recruitment seems to work through private networks. They are deeply covert and so not easy to track. Cells are not necessarily connected, but there are common factors: 80 percent of their members were born to Arab immigrant families.

Al Jazeera correspondent Tamer Al-Misshal goes to Molenbeek to investigate how the cell evaded detection. He uncovers a secret database of Europeans recruited by ISIL to fight in Syria and secretly meets a former ISIL fighter as well as one of the first Belgians to join the group.

He investigates ISIL recruitment in jails, as many recruits, including Abdelhamid Abaaoud, have a criminal background, and asks why the authorities failed to track Abaaoud in the criminal justice system.

Source: Al Jazeera News