Editor's Note: (The story was published before the bombings in Belgium took place on Tuesday. For latest news on those attacks click here.)
(CNN) A wave of relief swept through Brussels -- and the Belgian security services -- when Salah Abdeslam was detained Friday. France will finally be able to hold to account one man allegedly involved in the Paris attacks in November.
But in reality, Abdeslam's arrest is a footnote in the larger battle against terrorism in Europe.
Not that it's insignificant. A suspect who had been on the run after the worst terror attack in Europe in the modern era is now behind bars. According to Belgian officials, he may have been planning fresh attacks. If he talks (and his attorney says he is cooperating), he could provide intelligence on ISIS' methods and resources in Europe.
"It is of the utmost importance that Abdeslam was captured alive, because we can now try to reconstruct the entire scenario," Belgian State Security Chief Jaak Raes told Belgian network VTM on Sunday.
Abdeslam allegedly was one of at least 10 men directly involved in the Paris terror attacks. Most had entered Europe on fake documents after training in Syria. Several, including Abdelhamid Abaaoud and Samy Amimour, were well-known to the French authorities. But none was known to be back in France on November 13.
Question 1: How many others might have slipped into Europe under the radar?
Several hundred Europeans who have joined ISIS and other militant jihadist outfits are thought to have returned home. French terror expert Jean-Charles Brisard wrote in December's Sentinel, published by the Combating Terrorism Center, that 600 French citizens "are believed to be fighting alongside terrorist organizations abroad and 250 are believed to have returned" to France.
Raes told VTM: "We know that a number of people are possibly on their way to Western Europe, with the intention of conducting an attack. ... We need to stay very vigilant about that."
Similar warnings have come from British and French officials. They see the Paris attacks as the opening salvo in an evolving terrorism campaign. UK officials estimate that about half the 800 British residents thought to have gone to Syria and Iraq to join militant groups have returned home.
Paris demonstrated that ISIS operatives in Europe are able to finance extensive travel, "safe houses," acquire weapons and build workable suicide vests using the volatile high-explosive TATP. Abdeslam is said to have spent the best part of three months last year traveling around Europe in rental cars, allegedly meeting others involved in the Paris plot. And a woman who met Abaaoud immediately after the Paris attacks said he had boasted of some 90 ISIS operatives already in Europe.
ISIS is thought to have obtained hundreds and maybe more blank Syrian passports as it captured government offices. Many of those who have come to Europe have used high-quality forged documents. Last month, Eric Van der Sijpt, spokesman for Belgium's federal prosecutor, said the suspects in the Paris attacks "were known to travel through Europe using false identity cards."
Question 2: Why did it take so long to catch Abdeslam?
After the November attacks, Abdeslam left Paris and appears to have gone underground in the Belgian capital for 124 days despite being Europe's most-wanted man. The trail seemed to have gone cold long before last week. Both French and Belgian investigators speculated that Abdeslam had managed to return to Syria and expressed surprise that he was still in Brussels.
It appears that a network of sympathizers helped him hide in safe houses despite the risks of harboring such a wanted man. Such is the depth of the jihadist "community" in some parts of Europe.
Belgian Interior Minister Jan Jambon told CNN on Sunday that there was clearly "a logistics network and maybe also a network of supporters that ... are helping him and this is something we have to look at closer."
And Belgian Foreign Minister Didier Reynders said, "We have seen a new network of people around him in Brussels."
Uncovering these support networks is a monumental challenge for overstretched security services. For example, Belgian police shot dead a 35-year old Algerian during a raid Tuesday in the Forest neighborhood of Brussels. His name was Mohamed Belkaid. He had been in Syria with ISIS but was not on the intelligence services' radar.
Only after his death did Belgian security services establish he was one and the same as Samir Bouzid, an alias he had used to cross Europe and to wire cash in November to Abaaoud's cousin, Hasna Aitboulahcen, in Paris. She and Abaaoud both died in the police raid in the Paris district of St. Denis days after the November attacks.
Question 3: Who is still out there?
Ten men are in custody in Belgium over alleged links to the Paris attacks. But two potentially key suspected figures remain unaccounted for.
One is 31-year-old Belgian-Moroccan Mohamed Abrini, who allegedly drove with Abdeslam to Paris twice in the week of the attacks. The two had known each other as teenagers in the Brussels neighborhood of Molenbeek. Abrini has a criminal record for violent theft. He was briefly in Istanbul last summer and may have traveled into Syria. His younger brother was killed fighting for ISIS in 2014.
The French arrest warrant issued in November for Abrini described him as dangerous and probably armed. But there have been no reported sightings of him since November 12. Relatives insisted he was in Brussels the night of the attacks.
The other is Najim Laachraoui, who operates under the pseudonym Soufiane Kayal. He crossed from Hungary into Austria with Abdeslam and Belkaid (the man shot Tuesday) in September. "Kayal" and Belkaid were together when they made a money transfer to Abaaoud's cousin just before French police raided the St. Denis apartment where he was planning a second attack.
Both "Kayal" and Belkaid were older than most of the suspects, suggesting they may have had planning roles. Investigators say text messages were sent to the pair from the Bataclan on the night of the attack, one of which read "On est parti, on commence." ("We've left; we're beginning.")
"Kayal" also allegedly had rented a property in the Belgian town of Namur used by the cell ahead of the attack. Could he have been one of two men who escaped when Belgian police conducted the raid last week against the house in the Forest neighborhood?
Question 4: Who might be running these operations for ISIS?
There's no shortage of experienced jihadists from France, Belgium and elsewhere in Europe still thought to be in Syria. They include Salim Benghalem and Boubaker el-Hakim.
Both have deep roots among French jihadist groups going back more than a decade.
Hakim was involved with a Paris-based jihadist group in 2003-2004 and was close to one of the Kouachi brothers, who carried out the Charlie Hebdo attack last year. Released from a French jail in 2011 after serving a sentence for recruiting French jihadists to join the insurgency in Iraq, he left for Tunisia. He later claimed to have been involved in the 2013 killings of two secular politicians there. The same weapon was used in both attacks.
Hakim then traveled on to Libya and Syria. He is suspected of a role in planning the deadly attacks in Tunis and Sousse, Tunisia, last year. The United States designed Hakim a terrorist last year.
There are plenty of other foreign fighters in ISIS with intimate knowledge of the jihadist scene back home: French citizens such as Fabien Clain, who was associated with a plot in 2009 to attack the Bataclan theater in Paris and whose audio recording declared ISIS' responsibility for the November attacks.
Just what if any role these men might have in ISIS' "external operations" is unclear, but they and many others have the backgrounds to make a significant contribution.
Question 5: What is ISIS trying to achieve?
European intelligence services are convinced that ISIS is planning further terror attacks on the continent. It wants revenge for airstrikes on the "caliphate" amid growing pressure on the territory it has carved out in Iraq and Syria. It wants to spark a religious war between Muslims and Christians in Europe. And it knows it can infiltrate operatives into Europe using false documents amid the migrant influx.
In an interview with the ISIS online publication Dabiq last year, Hakim appealed to European militants to carry out attacks, saying: "You would wake up and fight them if you came here and saw what they do with their planes, how they terrify the women and children, and how they strive day and night to destroy this state."
As the Paris attacks showed, ISIS is intent on attacking "soft" targets rather than military or government facilities. Hakim encouraged such attacks with the message: "Do not look for specific targets. Kill anybody."
British Prime Minister David Cameron said in November that ISIS had "a dedicated external operations structure in Syria .... which seeks to target both the UK and our allies." And more recently a senior British police official, Mark Rowley, said ISIS had big ambitions for enormous and spectacular attacks in Europe.
Many analysts said they believe ISIS will encourage and sponsor more such attacks as its core territory is threatened. The growing sophistication of its European plots is ample evidence of that intent.
Question 6: Can Europe cope with the threat?
Modern Europe has been about expanding the European Union and erasing border restrictions. The Paris plotters took advantage of that, each of them crossing several borders without incident after reaching Europe. In addition to the travel from Hungary, Abdeslam accompanied another man, Ahmed Dahmani, on a ferry from Greece to Italy last summer. Dahmani has since been detained in Turkey.
Now the European Union is trying to tighten its external borders and hammer out a deal with Turkey on returning refugees who arrive in Greece. The task of tracking possible terrorists is made all the more difficult given the massive migration flows from Syria and Iraq and the economic crisis in Greece, where so many thousands arrive by boat.
European security agencies have not been famous for cross-border cooperation even as those borders have effectively vanished. In November, French officials were quick to point out that much of the planning for the Paris attacks had been done in Belgium -- although French Interior Minister Bernard Cazeneuve on Sunday praised the cooperation between Belgian and French security services in the hunt for Abdeslam.
However, even within borders there are rivalries among security agencies, with information-sharing patchy at best. Different spellings of names in a database handicap the tracking of individuals. And according to French counterterrorism analysts, monitoring just one individual can require some 25 agents.
ISIS has also been effective in disguising its communications, using encrypted messaging services such as Telegram.
The Paris attacks galvanized European security services and in France led to tougher anti-terrorism legislation and a surge in arrests. And yet the task is daunting. Jambon, the Belgian interior minister, told CNN on Sunday that "with every search of a house we find new information. We have to continue the trickle down of all this information."
There is a bigger long-term challenge, according to Jambon and many counterterrorism analysts: to counter the ISIS narrative that seduces so many young men.
"We are doing that -- but it will take some time before you have results in the field," Jambon said.