Several cities have housed Islamist cells, but the most active have been in Brussels and in the south-western suburb of Molenbeek in particular - an area with a high ethnic Moroccan population and a high rate of unemployment.
Several of the bombers and gunmen who targeted Paris last November, killing 130 people, had been living in Molenbeek. The main suspect not to die in the Paris attacks, Salah Abdeslam, returned to Belgium the day afterwards and managed to evade police until 18 March. He and an accomplice were captured alive, again in Molenbeek.
Many Belgians were expecting a response from jihadists. "I had certainly expected something else would take place, but not that it would happen on this scale," says Belgian jihadism expert Pieter Van Ostaeyen.