For three days last week, counterterrorism officials and experts from dozens of countries gathered in Brussels for a conference on a threat quietly gathering in the city around them.

They passed through an airport whose vulnerabilities had been assessed by Islamist militants. Some even stepped off a subway system already marked as a soft target. They heard a top Belgian official warn that the recent arrest of a suspect in last fall’s Paris attacks had exposed only the edge of a larger network.

The suspect, Salah Abdeslam, “was ready to restart something from Brussels” four months after he went underground, said Didier Reynders, the Belgian foreign minister. At the conclusion of the massive manhunt, authorities had “found a lot of weapons, heavy weapons . . . and we are seeing a new network of people around him in Brussels.”

The arrest of Abdeslam appears to have set off a race between security officials in Belgium and a terrorist cell that must have known that it had limited time to act. On Tuesday, the authorities lost.

The attacks, which killed at least 31 people and left about 250 others injured, were promptly claimed by the Islamic State, the terrorist group that has unleashed a series of external plots as its territory in Syria and Iraq has begun to shrink. Belgian officials described the devastation as a “black day,” darker than any the country had seen since World War II.

The carnage also exposed the extent to which Belgium has become the Western hub of a terrorist threat that has spread from the Islamic State’s strongholds in Syria across the Middle East and deep into Europe.

Belgium has seen a larger share of its Muslim population leave to fight in Syria than has any other Western country. The Molenbeek district of Brussels, the capital, is a particularly fertile breeding ground for militants, including several involved in the Paris attacks that killed 130 people last year.

Belgium was the first country in Europe to face an attack on its soil tied to the Islamic State: a shooting at a Jewish museum in Brussels that killed four people nearly two years ago. But despite that early wake-up call, the carnage Tuesday showed how woefully vulnerable Belgium remains.

“It’s kind of astonishing how hard it is for bureaucracies to be galvanized without direct experience of a major terror attack,” said Daniel Benjamin, a former top counterterrorism official at the State Department. “The tragedy is that country after country has had to learn this the hard way,” he said, describing the Islamic State’s support network in Belgium as “probably the most fully developed terror infrastructure in Europe.”

A small nation of 11.2 million, Belgium has had at least 470 of its citizens enter Syria to join the fighting since the civil war began there four years ago. That is triple the number of suspected fighters who have attempted to get to Syria from the United States. And the ratio of Belgian fighters – about 45 for every 1 million citizens – is more than twice that of its neighbor France.

Even before Tuesday’s attacks, those statistics made counterterrorism officials concerned that Belgium would face a dangerous “boomerang effect.”

“When you have these large numbers of foreign fighters, ISIS can cherry-pick the best ones to give them training and dispatch them to their home country to carry out attacks,” said Bruce Hoffman, a terrorism expert and director of Georgetown University’s Center for Security Studies. “Even if ISIS dispatches only 10 percent of these fighters, you’ve got the foundation of a potentially highly effective terrorist cell and support network.”

Initially, exporting violence did not appear to be one of the Islamic State’s priorities. For months after its declaration in 2014 of a restored caliphate, its energies seemed focused on expanding and controlling its territory in Syria and Iraq.

But U.S. officials and experts believe that the Islamic State has shifted its strategy over the past year as it has lost territory and momentum under a barrage of U.S.-led airstrikes and ground operations by Western-backed militias.