Here are some of the most significant twists and turns the investigation has taken and what's ahead.
Raids and arrests
Planning more terror
* In February, a laptop recovered by investigators in Brussels included a list of targets in Paris such as the financial district and a Catholic association, according to TV news station BFM. Over the past several months, many more pieces of evidence have been recovered at various locations, including explosives, equipment to make explosives and an ISIS flag.
Victims identified, mourned
Identifying Brussels bombing victims has been a long and difficult process. The victims spanned 40 nationalities.
A reopening for healing
At Brussels Airport on Sunday there was a brief opening ceremony before the departure hall began business.
"Sunday is an important day in the recovery of Brussels Airport. The doors of the departure hall to passengers will open again. The fact that this is possible in less than 40 days after the attacks is without any doubt a technical and operational tour de force that took an enormous effort of hundreds of people," said Arnaud Feist, CEO of Brussels Airport Company.
Feist called the moment an important turning point for the entire airport community and for Belgium. "For many of them it will be a difficult moment, but they can count on the support of our large airport family," he said.
Feist noted that in the immediate few months, "It will be very important to rebuild the image of our country abroad."
The airport is able to serve about 80% of the passengers it served before the attacks but Feist said it hopes to get fully up to pace by mid-June. Passengers are advised to allow at least three hours before their flights.
CNN's Lindsay Issac and Vasco Cotovio contributed to this report.
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