PARIS — Since the November attacks in Paris, the Belgian authorities have conducted dozens of raids, combed whole neighborhoods for well-known militants and even locked down the capital for days, all part of promises to step up efforts to root out jihadis.
Yet none of that evidently disrupted plans for the attacks on Tuesday at Brussels' main international airport and a subway station in the heart of the capital of the European Union.
The new attacks again underscored not only the weaknesses of Belgium's security services, but also the persistence and increasingly dangerous prospect of what several intelligence experts described as a sympathetic milieu for terrorist cells to form, hide and operate in the center of Europe.
The attacks have set off a new round of soul-searching about whether Europe's security services must redouble their efforts, even at the risk of further impinging civil liberties, or whether such attacks have become an unavoidable part of life in an open European society.
At the very least, they have exposed the enduring vulnerability of Europe to terrorism in an age of easy travel and communications and rising militancy.
Even before Belgian authorities captured Salah Abdeslam on Friday for his suspected role in the Nov. 13 Paris attacks, which killed 130 people, they had detained or arrested scores of suspects directly or peripherally connected to what they described as a terrorist network linked to the Islamic State.
But despite the success in arresting Abdeslam, Belgium continues to present a special security problem for Europe.
The country of just 11.2 million people faces widening derision as being the world's wealthiest failed state — a worrying mix of deeply rooted terrorist networks; a government weakened by divisions among French, Dutch and German speakers; and an overwhelmed intelligence service in seemingly chronic disarray.
It is also home to what Bernard Squarcini, a former head of France's internal intelligence, described as "a favorable ecosystem: an Islamist milieu, and a family milieu," which played an important role in sheltering Abdeslam and also perhaps in Tuesday's attacks.
"It shows that they were in a neighborhood that can shelter cells for months, because it is a neighborhood that is favorable to them," he said, referring to Molenbeek, the Brussels district where the Paris attackers lived and where Abdeslam was able to hide among family and friends.
The cultural code of silence in the heavily immigrant district, as well as widespread distrust of already weak government authorities, has provided what amounts to a fifth column or forward base for the Islamic State.
For weeks, intelligence operatives had warned that the next major terrorist attack on European soil was simply a matter of time. Even before Tuesday, Squarcini predicted that "there will be an even more serious attack," because, he said, "there are already the people in place."
Indeed, the presumed orchestrator of the Paris attacks, Abdelhamid Abaaoud, who lived in Molenbeek, boasted to his cousin before he was killed that "90" operatives were dormant, ready for another attack.
Some security and intelligence experts saw Tuesday's blasts as proof that Europe's open societies, even under states of emergency, will never be risk-free.
But the risks are fatally compounded, some said, by European-wide failures in intelligence sharing and the weakness of a Belgian intelligence service that Squarcini said lacked the capacity to pick up the "weak signals" of emerging plots.
"The Belgians are too limited to be able to treat several objectives at once," Squarcini said in an interview weeks ago.
"After a weekend of mutual congratulations" over the arrest of Abdeslam, he said Tuesday, "manifestly we didn't see the second wave."
But political and social failures have allowed militant cells to become deeply rooted, experts warned, and they were equally or even more worrying. Belgian officials spent weeks looking for Abdeslam, yet failed to turn up Tuesday's bombers.
"The mode of action was structured and agreed," said Ralf Jaeger, interior minister in North Rhine-Westphalia, a German state next to Belgium. "That presumes the formation of a cell. And that is what is frightening: that such a cell could not be discovered."
Those who are in place in Europe may now possess improved bomb-making skills and tactics, which can be adapted easily to additional security measures put in place by police and government authorities.
For instance, striking the check-in counter at the Brussels airport inflicted serious casualties and disrupted air travel while circumventing the millions spent on added security screening before passengers board planes.
Squarcini said airport security may now have to be revised Continent-wide, to take in even the approach to check-in counters — as is already the case in some parts of the world.
Others emphasized that progressive layers of new security measures can go only so far. Absent a military-style occupation, the threat from a well-established network with some degree of local complicity can never be completely forestalled, experts said.
"This shows the limits of the actions you can undertake in a state of emergency," like the one Belgium had in place for weeks, said Philippe Hayez, a former official with the DGSE, the French external intelligence service.
"These are time-specific, superficial," added Hayez, who has written extensively on Europe's intelligence challenges. "But unless you occupy it militarily, you don't hold a town just by circulating police cars. We're talking about guerrilla terrorism. And there's a population that's complicit."
That complicity may be most worrying, he and others said.
"We are paying for our naiveté," said Jacques Myard, a French parliamentarian who sits on his country's intelligence oversight committee. "It's not a weakness in intelligence. It's a weakness in society."
"The sleeper cells have been there, and they are well implanted," said Myard, a member of the conservative Republican party. For two years, the intelligence services "have been telling us: We've never seen such an influx" of terrorist operatives.
It was unclear whether Tuesday's bombings were a response to Abdeslam's arrest, or long in the works. In either case, said Alain Juillet, who helped reorganize the French external intelligence service as a top official there, "it's not surprising."
"That's the only thing one can say," he said. "We can easily see that Belgium has become a hub."
"So that when you arrest someone, there will be a reaction," Juillet said, referring to Abdeslam.
"All of this is to say that the implantation of the network is more firm than we thought," Juillet added. "The police were efficient — and yet this happened. So, there is a very strong implantation in Belgium."
But the fatal paradox for Europe is that on a border-free Continent, such problems play out transnationally. One country's failures are necessarily amplified.
Now the problems in Belgium are threatening not only lives across Europe, but also the Continent's experiment at integration. Whether the European Union, with its commitment to open borders, is strong enough to withstand the strains on top of years of economic crisis already is an ever more open question.
"It seems that the clear targets of the attacks — an international airport, a metro station close to EU institutions — indicate that this terrorist attack is not aimed solely against Belgium," Germany's interior minister, Thomas de Maizière, said at a news conference in Berlin. "But against our freedom, freedom of movement, mobility and everyone in the EU."