Intelligence services in Belgium and other European nations are working round the clock to identify the organisers of the latest terrorist attacks in Brussels, which claimed the lives of 31 people.
Yet even at this early stage, two grim conclusions are obvious: Europe has enough terrorist cells to make violence and bloodshed a persistent threat to the continent for years to come, and new security measures will have to be put in place to protect Europe's public spaces.
The bombing of the Belgian capital's airport and its subway system, for which the Islamic State in Iraq and Syria (ISIS) claimed responsibility, are almost certainly the terrorist network's response to last week's arrest by the Belgian authorities of Salah Abdeslam. The French national of Moroccan descent is alleged to have been one of the logistical masterminds of last November's terrorist strikes in Paris which killed 130 people.
It may take some time to find out if the Brussels attacks were intended as a message of defiance from ISIS, or whether the bombings were brought forward because ISIS commanders feared they may be exposed by Abdeslam, who is apparently cooperating with police.
But what remains clear is that the Brussels attacks were in the making for months: suicide bombers had to be trained, targets had to be picked and explosives prepared. So, at the very least, the bombings indicate that terrorist organisations continue to thrive in Belgium, notwithstanding increased vigilance by the authorities.
That is not surprising, for Belgium has long been known as a terrorist hub. Its Muslim community, around 8 per cent of the country's 11.2 million-strong population, is heavily concentrated in the capital, where Muslims make up at least a quarter of residents. That gave rise to enclaves such as Molenbeek, infamous as the favourite recruiting ground for terrorists. On a per capita basis, Belgium has contributed more volunteers to terrorism in the Middle East than any other nation in Europe. Successive Belgian governments ignored this threat, or portrayed it as beyond the country's abilities. "We don't have control of the situation in Molenbeek at present," said Interior Minister Jan Jambon last year, an unfortunate statement for a domestic security chief to make, even if true.
Nor was it particularly clever for Belgian Prime Minister Charles Michel to allow himself to become part of the media circus last week when Abdeslam was arrested, or to provide a running public commentary on the interrogation of that terror suspect. Both events raised unrealistic expectations about what the Belgian authorities can do to defeat terrorism.
For the reality is that the high frequency of attacks in Europe and the ease with which terrorists acquire weapons and explosives indicate that the menace remains great. And it may be increasing: the more ISIS is squeezed on the ground in the Middle East, the more incentive it will have to mount spectacular operations in Europe. Furthermore, those terrorist volunteers who return to Europe are battle-hardened and ruthless.
The Brussels tragedy also raises some fundamental questions about the need for additional security checks. It is generally accepted that underground train stations are impossible to control: Their whole purpose is to provide constant and instant access to large numbers of people, so apart from discreet counter-terrorism surveillance with face-recognition software, there is not much more to be done.
But airports could be offered better security, since most nations already have contingency plans to protect the perimeters of their airfields by searching passengers before they enter terminals. Turkey, for one, already does so, as does Japan, at least for its Tokyo airports.
And Israel operates a quadruple airport security system, which checks passengers well before they approach terminals, checks them again as they enter terminals, and then scans them at check-in desks and just before boarding. However, the Israeli model works only because it is ultimately based on racial profiling: Jewish citizens of the state are waved through, so delays imposed on the public are not considerable. But in Europe such racial profiling is unthinkable and unworkable, and disruption to air services will be great. Yet that may well be one consequence from this week's Brussels carnage.
Ultimately, however, the real hope of containing terrorism in Europe lies in allocating more resources to intelligence services, plus better and more efficient coordination between national law enforcement agencies.
For years, European leaders used to respond to terrorist attacks by vowing that violence will not change their continent's way of life. No such promises are made now.