The European Union's capital awoke under guard after 34 people were killed and more than 200 wounded in Tuesday's attacks. The Islamic State group, which was behind the Paris attacks, claimed responsibility for the Brussels bombings, which laid bare Europe's vulnerability to a group trying to spread violence well beyond its bases in the Middle East.
Police conducted raids overnight and circulated a photo of three men seen at the airport wheeling trollies that presumably contained explosives-filled suitcases.
Belgian state broadcaster RTBF identified two of the attackers as brothers Khalid and Brahim El Bakraoui, and said they are believed to have blown themselves up. According to the report, which did not say who its sources were, Khalid El Bakraoui had rented an apartment that was raided last week in an operation that led authorities to top Paris attacks suspect Salah Abdeslam.
One of the men pictured at the airport is at large. Authorities have not identified him, but Belgian newspaper DH reported that he might be Najim Laachraoui, whom Belgian authorities have been searching for since last week as a suspected accomplice of Abdeslam.
Laachraoui is believed to have made the suicide vests used in the Paris attacks, a French police official told The Associated Press, adding that Laachraoui's DNA was found on all of the vests as well as in a Brussels apartment where they were made. The official spoke on condition of anonymity because he was not authorized to discuss an ongoing investigation.
Abdeslam was arrested Friday in the Brussels neighborhood where he grew up, a rough place with links to several of the attackers who targeted a Paris stadium, rock concert and cafes on Nov. 13. Those attacks killed 130 people and terrified Europe.
A Belgian official working on the investigation told the AP that it is a "plausible hypothesis" that Abdeslam was part of the cell linked to the Brussels attack. The official spoke on condition of anonymity because he was not authorized to publicly discuss an ongoing investigation.
French and Belgian authorities have said in recent days that the network behind the Paris attacks was much larger than initially thought — and developments this week suggest the same group could have staged both the Paris and Brussels attacks.
Belgium's justice minister said Wednesday that the country will remain at its highest terrorism threat level until further notice. That level means there's a threat of an "imminent" attack.
The airport and several Brussels metro stations remained closed Wednesday. Security forces stood guard around the neighborhood housing the headquarters of European Union institutions, as nervous Brussels residents began returning to school and work under a misty rain.
As befits an international city like Brussels, the foreign minister said the dead collectively held at least 40 nationalities.
"It's a war that terrorism has declared not only on France and on Europe, but on the world," French Prime Minister Manuel Valls said Wednesday on Europe-1 radio. Valls, who planned to visit Brussels later Wednesday, urged tougher controls of the EU's external borders.
"We must be able to face the extension of radical Islamism ... that spreads in some of our neighborhoods and perverts our youth," he said. The Paris attackers were mainly French and Belgian citizens of North African descent, some from neighborhoods that struggle with discrimination, unemployment and alienation.
In its claim of responsibility, the Islamic State group said its members detonated suicide vests both at the airport and in the subway, where many passengers fled to safety down dark tunnels filled with hazy smoke from the explosion. IS warned of further attacks, issuing a statement promising "dark days" for countries taking part in the anti-IS coalition.
European security officials have been bracing for a major attack for weeks and had warned that IS was actively preparing to strike.