An empty apartment block on a quiet street turned out to be the perfect place for the three suspected Brussels attackers to prepare the home-made nail bombs used in Tuesday's airport and metro attacks, which killed at least 31 people.
In a building undergoing renovation, there were no near neighbors to notice them taking in large quantities of strong-smelling household chemicals, as well as a suitcase of nails, to concoct an unstable white explosive powder known as TATP (triacetone triperoxide) that they later used in their attacks.
"Even if someone had stopped them, they could have said the materials were for renovation," said Hassan Abid, an official at the local town hall, who was trying to determine why authorities had no knowledge of the men living illegally on the fifth floor.