BRUSSELS • As intelligence experts and officials took stock of what they have learnt since the assaults in and around Paris on Nov 13 last year, in which 130 people were killed, several things have come into focus.
The scale of the Islamic State in Iraq and Syria's (ISIS) operations in Europe is still not known, but it appears to be larger and more layered than investigators at first realised - if the Paris and last month's Brussels attacks are any model, plotters will rely on local criminal networks in addition to committed extremists.
Even as the United States, its allies and Russia have killed leaders of ISIS and have rolled back some of its gains on the battlefields of Iraq and Syria, ISIS appears to be posing a largely hidden and lethal threat across much of Europe.
When Belgian prosecutors announced that Mohamed Abrini, one of the men arrested last Friday, had confessed to being the mysterious third man in the Brussels Airport bombing, it seemed to be a victory.
But it also underscored the monumental challenges that extend across borders, for Abrini was also a suspect in the Paris attacks.
There are almost certainly other cells that are active in non-French-speaking countries and that have not yet surfaced. Britain, Germany and Italy are thought to be high on the list of ISIS targets.
It adds up to a long road ahead in Europe for law enforcement and intelligence agencies but also for citizens who are having to learn to adapt to an array of new security precautions, especially in public places.
"We are not finished yet with the job of finding everyone who is in this big network of Paris and Brussels," said Mr Jean-Charles Brisard, the head of the French Centre for the Analysis of Terrorism in Paris.
"Every time progress is made, we add another few people to the list of people we are looking for."
It is sobering to look at the number of people believed to have some connection to the Paris and Brussels attacks: 36 are suspected of being active participants to varying degrees in organising or carrying them out. Of those, 13 are dead, and most of the rest are in custody.
A handful have been released but subject to conditions, such as daily check-ins at a police station. Others are probably lying low or on the run.
What worries investigators is that many of the participants in the Paris-Brussels network were recruited by a preacher in the Brussels district of Molenbeek, Khalid Zerkani.
He was tried twice in Belgium, accused of recruiting more than 50 young men to join the fight in Syria and helping to finance their journey to the Middle East.
"There are still many people involved who were part of the Zerkani network, who were convicted in absentia - at least five to 10 - and we don't where they are or what they might do," Mr Brisard said.
While they could turn out to be minor players, they could also emerge as able organisers of new assaults.
Among Zerkani's recruits, prosecutors say, were Abdelhamid Abaaoud, the on-the-ground commander of the Paris attacks, and Reda Kriket, who was arrested on March 24 in a suburb of Paris.
Kriket has been accused of planning an attack in France involving "an arsenal of weapons and explosives of an unprecedented size", said Paris prosecutor Francois Molins.
From the ammunition and material found, it appears that a highly lethal attack was averted.
Four men in touch with Kriket who were arrested in Rotterdam in the Netherlands had in their possession 45kg of ammunition, according to Dutch public broadcaster NOS.
"Other Islamic State cells are highly likely to be in existence across Western Europe, preparing and organising further operations, and awaiting direction from the group's central leadership to execute," said Mr Matthew Henman, the head of IHS Jane's Terrorism and Insurgency Centre in London.
NEW YORK TIMES