More than 400 Belgian citizens are known to have left to fight for the Islamic State of Iraq and Syria (ISIS) in Syria -- the highest per capita number in Europe.
Salah Abdeslam is described as "dangerous" and Belgium has issued an international warrant for his arrest. He's believed to have rented a black Volkswagen Polo that was found near the Bataclan concert venue.
One of his brothers blew himself up with a suicide vest in the Paris attacks.
A third brother, Mohamed, says his family could not understand why they did it.
"We are indeed thinking of the victims, the families of the victims," he said. "But you must also understand we have a mother, we have a family and he is still her son."
A U.S intelligence source told CBS News that U.S. intelligence has identified Abdelhamid Abaaoud as the "mastermind" of the Paris terror plot and the key ISIS operative "for external operations in Europe and is operating in Syria."
The source also confirmed that that Salah and Ibrahim Abedeslam along with Ahmad Al-Mohammad and Samy Amimour travelled to Syria.
Abaaoud has been linked to a failed terrorist plot in Belgium and an attempt to gun down passengers on a high-speed train to Paris that was foiled by three Americans.
Abaaoud grew up in Molenbeek, an impoverished Brussels neighborhood where the jihadist and criminal worlds meet. It is a terrorist's dream and a security nightmare -- experts call it a "one-stop shopping" for drugs, explosives and automatic weapons.
Belgian Foreign Minister Didier Reynders called for better cooperation among European security agencies.
"The intelligence is the only possible way to block some possible terrorist activities before the terrorist attack," Reynders said.