Belgian police have issued a wanted notice for a young man who was caught on CCTV at Zaventem airport alongside two others who, investigators said, later blew themselves up in the terminal, killing at least 10 people.
Officials said about 20 died on the train close to European Union institutions. Islamic State have claimed that was also a suicide attack. The tolls were vague because of the carnage at both sites.
As the city began to emerge from a day of lockdown, a major police search operation was still under way after dark in the northern borough of Schaerbeek.
Investigators said they had found a nail bomb and an Islamic State flag in an apartment.
At Zaventem airport, the suspects were photographed pushing bags on trolleys and witnesses said many of the airport dead and wounded were hit mostly in the legs, possibly indicating blasts at floor level.
Two of them seem to have committed suicide attacks. The third, wearing a light-coloured jacket and a hat, is actively being sought," Federal Prosecutor Frederic Van Leeuw told a news conference.
A government official said the third suspect had been seen running away from the airport building. Police later found and detonated a third explosive device at the airport.
Belgian authorities were still checking whether the attacks were linked to the arrest of Salah Abdeslam, although Belgian security experts said the level of organisation involved suggested they had been in preparation for more than just a few days.
After questioning Abdeslam, police issued a wanted notice on Monday, identifying 25-year-old Najim Laachraoui as linked to the Paris attacks. The poor quality of Tuesday's CCTV images and of the Laachraoui wanted poster leaves open whether he might be the person caught on the airport cameras.
A lockdown imposed after the attacks was eased and commuters and students headed home as public transport partially reopened.
In the United States, major transportation hubs have been placed on alert, and part of Denver International Airport's main terminal was evacuated, following the suicide bombings in Brussels.
The evacuation area at Denver encompassed the west side of the terminal, levels five and six, but flight operations were able to go on while the potential threat, described by police as a "suspicious package," was investigated, the airport said.
Vehicle traffic was halted on the west side of the Denver terminal, but the east side remained open to passengers, the airport said.
Major US airports and other transportation facilities were put on alert following attacks at Brussels Airport and a subway station on Tuesday for which Islamic State, the militant group that has seized large swaths of Iraq and Syria, claimed responsibility.
Large numbers of uniformed police officers and National Guard members in fatigues and carrying long weapons patrolled New York's John F Kennedy International Airport.
Denver airport said on its Twitter feed that airline ticket counters affected by the incident there included American Airlines, Aeromexico, Air Canada, Lufthansa and British Airways.
"Flight delays are possible," it said.
A Denver airport spokesman, Heath Montgomery, said earlier in the day that no additional security measures were being taken there, although he added that airport officials remained in contact with the US Department of Homeland Security and Transportation Security Administration.
"Especially in light of the events today, we share a heightened sense of awareness," Montgomery said.
US President Barack Obama and Democratic presidential election contender Hillary Clinton vowed to do more to confront militants, while Republican presidential front-runner Donald Trump called for tighter border security and suggested US intelligence services could use torture to head off future attacks.
The Obama administration was expected to impose tighter security measures at US airports following the Brussels airport attack, which occurred in a public hall outside of the security check area.
Delta Air Lines Inc, United Continental Holdings Inc and American Airlines Group Inc cancelled or rerouted flights as a result of the Brussels attacks.
Reports say that nine Americans have been killed in the attacks in Belgium.