The bombs exposed the limitations of police and intelligence work in Belgium even in a European capital already on high alert for terrorism. There have been long-standing failures to penetrate groups of Islamic extremists living in Brussels.
Is there significance to the timing?
Isis (Islamic State) militants claimed responsibility for the atrocities in Brussels. The bombings came four days after Belgian police arrested Saleh Abdeslam, a Brussels-born Frenchman who prosecutors believe helped perpetrate the November massacres in Paris. Abdeslam was hiding out in Molenbeek, a heavily Muslim neighbourhood of Brussels that's seen repeated raids by armed police since the Paris attacks. Experts have generally concluded that the new bombings show there was a wide terror network in place. Writing in the New Statesman, Shiraz Maher of King's College, London, said: "It is unlikely a cell would have been able to mobilise so quickly and build several viable devices [after the arrest]. Much more worrying is that [the] attack suggests the existence of a broad terrorist network in Belgium - one that was already primed and ready to attack, long before police caught up with Abdesalam".
What does it mean for Isis?
Max Fisher, a foreign policy expert at Vox says: "When Isis began to lose its caliphate - its territory chipped away by Kurdish groups, Shia militias, the Iraqi Army, and US-led airstrikes - [its] narrative of victory and invincibility began to collapse as well. If it wanted to maintain its ideological strength, from which it partially derives its military strength, it would need to find a new way to prove itself, a new way to capture global headlines and attention. It found the answer in coordinated terror attacks against civilians."
How difficult is it to keep tabs on jihadists?
French security experts have said it takes 10 to 20 agents to keep complete tabs on a single individual. More worrying is the unbalanced arithmetic inherent to modern counter-terrorism: While the police have to win every time, attackers need only to win once to cause carnage. "You're dealing with a very big problem that has quite strong roots in this country, and that the Belgians are having difficulty dealing with," said Raffaelo Pantucci, director of international security studies at London's Royal United Services Institute. Even with suspected extremists under tight watch, "you don't know that you're not just looking at a partial piece of the picture," he said. "People underestimate the amount of resources it takes to surveil someone," Scott Stewart, the vice-president of tactical analysis at security firm Stratfor and a former US State Department special agent, says. Even with a small number of suspects, "you have shifts, you have days off. You need technicians, translators. It just becomes overwhelming."
Why is surveillance particularly difficult in Belgium?
Belgium's main intelligence agency, State Security, has just 500 to 600 staff, according to local press reports. Belgian politicians have also lamented the fragmentation of policing, with six police departments responsible for law and order in Brussels, a city of just over a million residents. Coordination efforts are also hindered by long-standing, nationwide tension between French- and Flemish-speaking communities.
Does Belgium have a major problem with jihadists?
With an estimated 440 citizens fighting with militant organisations in Syria and Iraq, the country of about 11 million is the largest per-capita European source of Islamic extremists, according to a 2015 study by the International Centre for the Study of Radicalisation.
What previous instances have there been?
In January, police killed two suspected terrorists and arrested a third in Verviers, near Liege, preventing what might have been a "major attack." In 2013 a gunman who had spent time in Syria killed four at the Brussels Jewish Museum, and last year passengers overpowered a man wielding a Kalashnikov rifle on a high-speed train that had just departed the capital.
Are there wider issues?
Yesterday's attacks took advantage of vulnerabilities common to every modern society, not just Belgium, and particularly in the European Union, whose cornerstone is freedom of movement around the continent. The airport explosions occurred well outside the security cordons for departing passengers, and like most cities Brussels has no security checks at subway entrances. It's not easy to see how either type of location could be better protected, security experts said. External checkpoints at airports, for example, would just present a new target. "If you have a motivated, dedicated person who wants to kill someone it's quite easy," Stewart said. "There are soft targets everywhere."