BRUSSELS — The stench of chemicals emanating from the sixth-floor apartment 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 odor leaking from their curiously heavy luggage as he drove them to Brussels Airport.

But not until 7:58 on Tuesday morning did these and other strange and, at least in retrospect, alarming dots come together to form a clear picture of what had been going on for more than two months in the dilapidated but spacious top-floor apartment at 4 Max Roos Street in the Brussels borough of Schaerbeek.

It was then that two homemade bombs — confected from malodorous and highly volatile chemicals in the living room of the apartment — exploded in the check-in area of the airport, followed an hour later by another at a busy subway station. Together, the attacks killed 31 people.

A third bomb was found unexploded at the airport, but the two that were detonated blew holes in the roof and maimed scores of people as they waited to check their baggage.