BRUSSELS • The stench of chemicals emanating from the sixth-floor apartment at 4 Max Roos Street made the owner of the building gag. Other odd happenings at the mostly empty housing block in northern Brussels prompted an anxious resident in the area to alert the police.
A taxi driver who picked up three young men at the block smelled a noxious odour leaking from their curiously heavy luggage as he drove them to the airport in Brussels.
But not till the bombs went off last Tuesday - killing 28 people and the three suicide bombers - at the international airport and a rush-hour metro train, did these dots come together to form a clear picture of what had been going on.
How the lengthy preparation for the attacks could have gone undetected has stirred dismay and anger from politicians and also the public, as well as from allies in Europe and beyond.
The airport was yesterday still closed, a huge and macabre crime scene instead of a global crossroads and the main entry point to the "capital of Europe", a city that houses the headquarters of the European Union and Nato.
Acting with uncharacteristic - and still unexplained - swiftness, Belgian security forces sealed off the area around the apartment in Schaerbeek within 90 minutes of the airport attack. The authorities attributed their speedy reaction to a tip-off from the taxi driver.
But the driver was said to have alerted the police only after a photograph of the bombing suspects was released hours later.
"There were investigations before and after the events" of last Tuesday, Mr Alexandrino Rodrigues, the owner of the building, said in an interview, suggesting that the police had suspicions before the attacks and had not entirely ignored the report from a concerned neighbour.
With its large communities of often poor and poorly integrated Muslim immigrants and its own deeply rooted divisions of language, geography and politics, Belgium finds itself at the centre of Europe's struggle with terrorism and a glaring example of the obstacles blocking the way.
"Why such repeated dysfunctions?" Mr Marco Van Hees, a member of the Belgian Parliament, asked the interior minister and two other ministers who were summoned on Friday to explain the failure.
"We are certainly not dealing here with just a glitch, a little bug, but a deep structural problem."
That Belgium has a serious problem with militancy has been clear for years, particularly since January last year, when the police raided a terrorist hideaway in the eastern town of Verviers and foiled what the authorities said was a major plot.
That success, however, masked rather than solved the problem, which exploded with brutal horror in Paris, when militants, many of them Belgians, killed 130 people with bombs made in the Brussels district of Schaerbeek.
It was the same area that would later house the bomb-making workshop on Max Roos Street, where two of the suicide bombers in the Brussels attacks, brothers Ibrahim and Khalid el-Bakraoui, had lived since the beginning of the year.
How was it possible, members of Parliament asked, that Ibrahim and Khalid had managed to go undetected for so long? And all this despite a record of violent crime in Belgium and, in the case of Ibrahim, a clear warning from Turkey in June that he was on his way back to Europe after being arrested as a suspected terrorist while on his way to Syria?
In a sign the Belgian authorities had the el-Bakraoui brothers in their sights before they carried out their attacks, interrogators presented Salah Abdeslam, the only known survivor among the terrorists responsible for the Paris attacks and who was captured in Belgium on March 18, with photographs of the siblings after his capture and asked if he knew them.
According to excerpts from the March 19 interrogation obtained by Le Monde, Abdeslam firmly denied knowing the men who three days later would subject Brussels to its bloodiest attack since World War II. The Belgians dropped the matter.
Mr Rodrigues, who owned the building that the el-Bakraoui brothers had dwelled in, said the police officer in charge of his district had stopped by the building at least twice to check whether the names of residents were properly listed, but never entered their apartment because the brothers did not list their names.
"It took him three months to figure out that people were living" on the top floor, Mr Rodrigues said, adding: "But it was too late. That's how things work in Schaerbeek."
NEW YORK TIMES, REUTERS