It has also highlighted the dangers posed by a well-developed underground jihadist pipeline that has made Belgium Europe’s biggest per capita contributor of fighters to Syria, and the fears of the potential havoc these extremists could sow upon their return.

Despite the attention focused on France since the attacks in and around Paris that killed 17 people, the proportion of young people who have left for jihad from this relatively small country has confronted the authorities here with an outsize domestic security threat that rivals that of its neighbor.

In a document released in October, a new Belgian government warned against the “danger of violent jihadism that threatens to spread in our society,” reporting that 350 Belgians had gone to Syria and that more than 70 of them had returned home.

Pieter Van Ostaeyen, a Belgian researcher who has kept close tabs on Syria-bound jihadists from Belgium, said the real number of Belgian fighters is closer to 450, less than half the number from France but still a very large contingent for a country of only 11 million people. Belgium, like France, has a large Muslim community that accounts for more than 5 percent of the population.