Police issued a wanted notice for a young man pictured pushing a laden luggage trolley at the airport where two others were suspected of blowing themselves up.
The coordinated assault triggered security alerts across Europe and drew global expressions of support, four days after Brussels police had captured the prime surviving suspect in Islamic State's attacks on Paris last November.
Belgian security officials made little comment but U.S. officials said that while the attacks may have been triggered by the arrest of Salah Abdeslam, the level of organisation involved suggested that they had previously been in preparation.
A witness said he heard shouts in Arabic and shots shortly before two blasts struck a packed airport departure lounge at Zaventem airport.
Belgian media published a security camera picture of three young men pushing laden luggage trolleys. Police later issued the same photograph, showing only one of the three, saying the unidentified man in a hat and wearing glasses was a suspect.
"If you recognise this individual or if you have information on this attack, please contact the investigators," the notice read. "Discretion assured."
Police operations were under way at several points in the city but a lockdown imposed immediately after the attacks was eased and commuters and students headed home as public transport partially reopened.
Islamic State issued a statement claiming responsibility: "We promise the crusader alliance against the Islamic State that they will have black days in return for their aggression against the Islamic State," the jihadist group said.
Belgium, home to the European Union and the headquarters of the NATO military alliance, has sent warplanes to take part in operations against Islamic State in the Middle East.
Austrian Horst Pilger, who was awaiting a flight with his family when the attackers struck, said his children had thought fireworks were going off, but he instantly knew an assault was underway.
"My wife and I both thought 'bomb'. We looked into each other's eyes," he told Reuters. "Five or 10 seconds later there was a major, major, major blast in close vicinity. It was massive."
Pilger, who works at the European Commission, said the whole ceiling collapsed and smoke flooded the building.
Security services found and destroyed a third bomb after two blasts at the airport killed at least 10 people and injured around 100, the provincial governor of Brabant Flanders said. Belgian media gave death tolls as high as 14 at the airport.
The metro station blast killed a further 20 people and injured roughly 130, according to a provisional toll from the national crisis response centre.
U.S. President Barack Obama led calls of support to Belgian Prime Minister Charles Michel after Brussels had gone into a state of virtual lock-down.
"We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism," Obama told a news conference in Cuba. "We can and we will defeat those who threaten the safety and security of people all around the world."
Michel spoke at a Brussels news conference of a "black moment" for his country. "What we had feared has come to pass."
The blasts occurred after the arrest in Brussels of a suspected participant the Paris attacks that killed 130 people. Belgian police and combat troops on the streets had been on alert for reprisal but the attacks took place in crowded areas where people and bags are not searched.
All public transport in Brussels was initially shut down, as it was in London during 2005 Islamist militant attacks there that killed 52. Authorities appealed to citizens not to use overloaded telephone networks, extra troops were sent into the city and the Belgian Crisis Centre, clearly wary of a further incident, appealed to the population: "Stay where you are".
Brussels airport will remain closed on Wednesday, its chief executive Arnaud Feist told reporters.
Public broadcaster VRT said police had found a Kalashnikov assault rifle next to the body of an attacker at the airport. Such weapons have become a trademark of Islamic State-inspired attacks in Europe, notably in Belgium and France, including on Nov. 13 in Paris.
Alphonse Youla, 40, who works at the airport, told Reuters he heard a man shouting out in Arabic before the first explosion. "Then the glass ceiling of the airport collapsed."
"I helped carry out five people dead, their legs destroyed," he said, his hands covered in blood.
Others said they also heard shooting before the blasts.
A witness said the blasts occurred at a check-in desk.
Video showed devastation in the hall with ceiling tiles and glass scattered across the floor. Bloodied bodies lay around.
Some passengers emerged from the terminal with blood spattered over their clothes. Smoke rose from the building through shattered windows and passengers fled down a slipway, some still hauling their bags.
Britain, Germany, France and the Netherlands, all wary of spillover from conflict in Syria, were among states announcing extra security measures. Security was tightened at the Dutch border with Belgium.
The blast hit the train as it left Maelbeek station, close to European Union institutions, heading to the city centre.
VRT carried a photograph of a metro carriage at a platform with doors and windows completely blown out, its structure deformed and interior mangled and charred.
A local journalist tweeted a photograph of a person lying covered in blood among smoke outside the station, on the main Rue de la Loi avenue which connects central Brussels with the EU institutions. Ambulances were ferrying the wounded away and sirens rang out across the area.
"WE ARE AT WAR"
“We are at war and we have been subjected to acts of war in Europe for the last few months," French Prime Minister Manuel Valls said.
Train services on the cross-channel tunnel from London to Brussels were suspended. Britain is advising its citizens to avoid all but essential travel to Brussels, Prime Minister David Cameron's spokeswoman said.
Security services have been on a high state of alert across western Europe for fear of militant attacks backed by Islamic State, which claimed responsibility for the Paris attack.
While most European airports are known for stringent screening procedures of passengers and their baggage, that typically takes place only once passengers have checked in and are heading to the departure gates.
European stocks fell after the explosions, particularly travel sector stocks including airlines and hotels, pulling the broader indices down from multi-week highs. Safe-haven assets, gold and government bonds rose in price.
The attacks appeared to be linked to the arrest of French citizen Salah Abdeslam - the prime surviving suspect for the Paris attacks on a stadium, cafes and a concert hall - who was captured by Belgian police after a shootout on Friday.
Belgium's Interior Minister, Jan Jambon, said on Monday the country was on high alert for a revenge attack.