Both men had criminal records, but Belgian officials initially said they were unaware of any ties to terrorism.
The Belgian federal prosecutor told a news conference on Wednesday that Ibrahim El Bakraoui, 29, one of two men who blew themselves up at Brussels airport, had left a will on a computer dumped in a rubbish bin near the militants' hideout.
Explosives were found in his appartment, leading investigators to suspect he was plotting another attack.
Turkey says it deported one of two brothers behind the Brussels bombing attack in 2015, and that Belgium ignored its warnings. European security officials identified another suicide bomber as Najim Laachraoui, a suspected bombmaker for the Paris attacks.
And Laachraoui, like the unidentified man seen wearing a white jersey at the Brussels airport on Tuesday, remains at large, a fugitive link in a chain still being forged.
The FBI has sent a "fly team" from its NY field office to assist Belgian authorities in their investigation of the terror attacks in Brussels, a law enforcement source told Fox News Wednesday.
Analysts said Laachraoui was believed to be a key bomb maker, and French media said he also played a major role in the terror attacks in Paris.
Brahim El Bakraoui who struck at the city's airport, said in the note, described by a prosecutor as a written testament, that he was "on the run" .
Belgian authorities had been looking for Laachraoui since last week, suspecting him of being an accomplice of top Paris attacks suspect Salah Abdeslam, who was arrested on Friday. Belgian officials, however, cautioned that it would have been hard for the terrorists to put together such a sophisticated attack on short notice.
Metro stations will gradually reopen with a heavy security presence, and the airport will reopen no earlier than Monday, officials said.
Police charge 2 people in Sydney with funding Islamic State
A Sydney schoolgirl has been arrested by counter-terrorism police for allegedly sending money to a terrorist organisation. A teenage girl and a 20-year-old man are accused of sending money to the so-called Islamic State from Australia.
In the case of Tuesday's attacks, Abdeslam's arrest may have been a trigger for a plot that was already far along.
At the core of the Belgian investigation is a photo taken from the airport's surveillance cameras showing three attackers walking side by side as they push luggage carts.
When police arrived at the Forest apartment on March 15, they had intelligence linking it to the Paris attacks but expected the property to be empty.
"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.
European Union interior and justice ministers are due to hold a crisis meeting in Brussels on Thursday to discuss their response to the bombings.
The prosecutor also confirmed that police had carried out raids following Tuesday morning's attacks, including at an address associated with the suspects after a tip-off by a taxi driver who drove the attackers to the airport.
In claiming responsibility for the attack, ISIS said it had a "secret cell of soldiers" dispatched to Brussels.
The fighters are organized into semiautonomous cells that are empowered to choose when, where and how they attack, according to the report, which cited European and Iraqi intelligence officials and a French lawmaker. The Brussels airport announced that it will remain closed to passenger flights for at least another day, right up to the start of the busy Easter weekend.