As hundreds of mourners gathered in the city in a show of solidarity with the 31 people killed and 270 injured, prosecutors said brothers Ibrahim and Khalid El Bakraoui had carried out attacks at Zaventem airport and Maalbeek station.
Bomb-making expert Najim Laachraoui was identified by police sources to AFP as the second airport bomber, while police stepped up a huge manhunt for a third airport attacker whose suitcase bomb failed to detonate.
All three suspects have been linked to the November attacks in Paris, which like the Brussels bombings were claimed by IS, underscoring the threat European nations face from the jihadist group.
US President Barack Obama urged nations to unite against terrorism and said wiping out IS was his "top priority."
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Prosecutor Frederic Van Leeuw revealed that Ibrahim El Bakraoui had left a desperate "will" on a computer that he dumped in a trash can, in which he said he felt "hunted" and added "I don't know what to do."
A third man in a hat and white jacket seen on CCTV footage with Bakraoui and Laachraoui pushing their bomb-filled bags on trolleys through the departure hall shortly before the attacks is still at large, Van Leeuw said.
Belgium has declared three days of mourning and on Wednesday hundreds of airport staff and their families carried candles and flowers in a silent march and vigil near Zaventem.
King Philippe, Prime Minister Charles Michel and European Commission chief Jean-Claude Juncker led a minute's silence outside the EU headquarters in Brussels, the city that is also home to NATO.
In the city's Place de la Bourse, where mourners have laid banners and candles, defiant applause broke out among the large crowd gathered to honour the dead, chanting: "Long live Belgium".