The Brussels attacks hint at a worrying 'iceberg' theory about terror networks in Europe
The attacks came days Saleh Abdeslam, a suspect in last year's Paris attacks, was arrested in Brussels, the capital of Belgium and de-facto captial of the European Union.
Clint Watts, a senior fellow at the George Washington University Center for Cyber and Homeland Security, noted on Tuesday that the Brussels attack are in line with an "iceberg" theory of terrorist plots.
That theory purports that, just as for every iceberg seen above water, the vast majority of its mass cannot be seen - "for every attacker, there are usually three to four additional people who helped facilitate the plot."
"That the eight attackers in Paris used more explosive belts than ever before seen in the West suggests a sizeable European terrorist facilitation network," Watts wrote for War on the Rocks in November.
He added: "The iceberg theory of terrorist plots suggests we should look for two, three, or possibly four dozen extremist facilitators and supporters between Syria and France. This same network is likely already supporting other attacks in the planning phase."
Belgian officials have long been aware of the existence of an ISIS-linked terror cell in Brussels, believed to be centered in the district of Molenbeek. Belgium's interior minister, Jan Jambon, has called Molenbeek "the capital of political Islam in continental Europe," and multiple suspects have been arrested there in connection to the Paris attacks.
Outside of Belgium, at least 18 people have been detained across Europe since November for their alleged roles in the Paris attacks, The New York Times reported last weekend.