Europe holds its breath: is ISIL's worst yet to come?
Airport and train station to remain closed as investigators assess damage from twin blasts that killed 31
Samla da Rosa was in the carriage next to the explosion, on her way to hear the head of the UN's Food and Agriculture Organisation speak in Brussels. She was reading an article on her mobile phone when her world changed.
The bomb that ripped apart the adjacent carriage at the Maelbeek metro station, about 100 metres from the headquarters of the European Commission, left 20 people dead and scores wounded. Da Rosa was among the first to see the awful carnage.
"I bumped against a woman who was very wounded," said the 54-year-old Brazilian. "She had burns on her face and hands. Her hair was burnt.
"Other wounded people were like zombies, walking without direction. It was a like a horror movie. There was an arm on the sidewalk in a black sleeve."
Da Rosa works in PR, is married to a Dutchman and has lived in Belgium for 19 years. She said survivors tried to comfort each other.
"They say you are born alone and you will die alone, but when you have an accident like that I didn't feel alone because the solidarity was so big. You are with people that you've never seen in your life. I hugged this young guy in front of me, and I was asking him, 'Why? Why are they doing this?'"
She is one of millions of people asking the same question - and who are demanding to know if crucial clues were ignored that might have prevented the awful events of last week.
The message from Turkey the day after the bombing was blunt. The Belgian embassy had been warned that a would-be jihadi had been sent back to Europe after trying to slip into Syria to fight and it had done nothing to stop him.
That man, Ibrahim el-Bakraoui, went on to blow himself up in Brussels Airport with a second suicide bomber, and soon afterwards his younger brother blew himself up in a metro carriage.
They killed 31 people and injured 300 in blasts so devastating that many of the dead may not be formally identified for weeks.
The failure to catch Bakraoui or track the brothers was the first in a string of basic intelligence mistakes that have led some to question whether Tuesday's bloodshed could have been avoided.
The week appeared to begin well for security forces, who had just captured Paris attacker Saleh Abdeslam after four months on the run. But the man they thought was the last surviving member of a broken terrorist cell turned out to be part of a much more sophisticated international network that was still active.
So the key question hanging over last week's tragedy is whether it will prove the bloody but final unravelling of the main Isis operation in Europe, or whether authorities are simply chasing another disrupted cell in a larger and more durable operation.
Even the French president has been circumspect about how fast intelligence teams can trace and dismantle a system forged in Syria and Iraq, and bolstered by family and childhood friendship at home, and which is apparently behind a string of terror attacks from the Charlie Hebdo shootings onwards.
Police were on the trail of the suspected bombers soon after they put out security camera footage of the men pushing luggage trolleys through the airport, which they had identified as suspicious partly because they were wearing gloves on only one hand. These are thought to have concealed their detonators.
A taxi driver called a tip-off line after recognising a group of difficult clients from that morning who had been unusually edgy about handling their large suitcases. The unnamed driver may have averted an even greater tragedy when he refused to take one piece of baggage because the car was overloaded.
A raid on the address he gave in the Schaerbeek district produced 15kg of explosives, detonators and a suitcase filled with nails and screws, along with vital clues to the identity of petty criminals Ibrahim el-Bakraoui and his brother Khalid. Other raids in the same street turned up a computer with a confused statement by Ibrahim. He described feeling "in a rush, not knowing what to do, being hunted everywhere, not being safe, and if this goes on, ending up in a cell".
His sense that authorities were on the militants' trail came as a bitter irony to some in Brussels, now on the highest alert. Grim details of missed clues and ignored intelligence trickled out as authorities chased associates of the men across France, Belgium and Germany.
The Bakraoui brothers, both Belgian nationals, had long criminal records, but officials had not detected any links with terrorism. It has emerged since that they had connections to the Paris terror cell, as had a second airport bomber: DNA linked Najim Laachraoui to two of the explosive belts used in the Bataclan concert hall.
By Thursday, the justice and interior ministers had both offered their resignations, although the prime minister refused to accept them.
The most dramatic revelation came from Turkish president Recep Tayyip Erdoan, who disclosed that Ibrahim el-Bakraoui had been expelled from the country in July last year, when Turkish authorities warned Belgian and Dutch counterparts of suspicions that he was a militant. That alert never got from Belgian diplomats to security forces at home. Nor had authorities chased Bakraoui for skipping parole after serving less than half of a nine-year sentence for armed robbery.
Then a local police force admitted that the key to finding Europe's most wanted man had lain unread for months in the filing cabinet of a Belgian police station.
The continent's top security officials had frantically pored over scant clues to where Abdeslam was hiding. But unknown to them, some time previously a local beat officer had worked out that a relative of Abdeslam was living at 79 Rue des Quatre Vents and thought to have been radicalised.
Three months later, with the report still unread, police finally descended on the apartment and found Abdeslam inside. His relative, Abid Aberkan, had served as a coffin-bearer for Saleh's brother and fellow Paris attacker Brahim just the day before the raid.
Many of the men behind the Paris and Brussels attacks are now dead or in custody. European police have rounded up around 20 people in France, Germany and Brussels. More than a dozen have died as suicide bombers or in stand-offs with police.
But Abaaoud, the man thought to be a key planner behind the Paris attacks, had boasted to a niece that he had brought around 90 militants back to Europe with him. Even allowing for boastful exaggeration, that is still many more than have died or been detained. In Belgium alone, there were thought to be at least 130.
It is difficult for authorities to distinguish between those who have left the Isis frontline because they have been disillusioned by the violence, from those who found it inspiration for atrocities at home.
The close-knit network of family and friends that produced so many of the attackers, then sheltered Abdeslam, has made the work of police particularly hard.
Community leaders and activists say they are trying to root out extremism.