The first victim to be identified is Adelma Marina Tapia Ruíz, who was killed during the attack at the airport, according to Peruvian state news agency Andina.
Originally from Peru, Tapia Ruíz had lived in Belgium for six years and was at the airport with her husband and twin 3-year-old daughters waiting to board a plane when the blasts went off.
About 8 a.m. local time, two explosions struck the departure lounge of Brussels Airport in Zaventem.
One was a suicide bombing, according to Belgian prosecutor Frederic Van Leeuw. The other remains under investigation.
One blast took place outside the security checkpoints for ticketed passengers and near the airline check-in counters, according to an airline official briefed on the situation. A bystander's video captured the horror of the aftermath, with people motionless on the floor and dust hanging in the air, obscuring visibility.
A third bomb was left at the airport but didn't go off, according to Belgian Interior Minister Jan Jambon.
Metro station: About an hour after the airport explosions, another blast rocked the Maelbeek metro station toward the end of rush hour. The station is in the heart of the city, where European Union institutions are based, a symbolic target for terrorists. NATO is also headquartered in Brussels.
Bystander video depicted passengers stepping down from the cars in a dark tunnel, with a child crying.
Suspects: Belgian authorities released an image of three suspects in Tuesday's Brussels Airport attack.
Two of them carried out suicide attacks, and a third one is believed to have been a guide to ensure the other two would carry out the attacks, according to Van Leeuw and experts.
That third individual, dressed in a light jacket and hat, is being sought, the prosecutor said.
Information on suspects in the metro station attack wasn't immediately available.
Belgian authorities have shared with security counterparts in the United States and elsewhere the names of several people they believe may have some role in the attacks, according to multiple U.S. officials. The names are being run through databases of suspected or known terrorists.
But the identities of the bombers aren't clear to U.S. authorities, either because Belgian authorities haven't completed identification or haven't shared that information with counterterrorism officials in the U.S., the officials said.
Investigators have used surveillance video to track the movements of the three men seen in the photo released, one U.S. official said. The three are seen leaving a taxi and moving through the airport, according to two officials.
The third man, dressed in white, left the airport after accompanying the two. That appeared to be planned, the officials said.
Raids: Police raids connected to Tuesday's attacks were happening in the Brussels region, with authorities looking for suspects linked to the explosions, Belgian broadcaster RTBF reported, citing judicial sources.
A taxi driver who drove the suspects to the airport told investigators the location where he picked up the three men. He recognized the men from the surveillance images, officials said.
Investigators were searching residences, but they weren't saying where, Van Leeuw said.
At least one police helicopter with a sniper hovered over the Schaarbeek neighborhood as darkness fell Tuesday. The chopper apparently provided cover for raids unfolding on the ground, analysts said.
"During a house search in Schaarbeek, investigators found a nail bomb, chemical products, and an ISIS flag," the federal prosecutor said in a statement.
Airport discovery: Belgian media reported a Kalashnikov assault rifle was found in the departure hall of the Zaventem airport.
Suitcase bomb? Investigators are looking at whether one of the airport explosions may have been caused by a bomb inside a suitcase, while the other was a suicide bombing, according to a U.S. official briefed on early evidence from the investigation.
This investigation is only just beginning and there's much that's not yet known, the source added.
Working theory about airport: Investigators suspect the attack was one of opportunity, targeting a large group of people in an area near airline ticket counters, two officials say.
There's no indication so far the attackers tried to go past the security screening checkpoint, but investigators will have to do more investigation, including reviewing surveillance footage, to be sure, the officials say.
Authorities have yet to provide details on what they think the attackers' strategy was at the train station.
"The Belgians have been sitting on a ticking time bomb," one U.S. counterterrorism official said.
U.S. intelligence officials say they weren't surprised about an attack in Brussels because there have been general concerns about terror threats, particularly in the wake of recent raids and the arrest of key Paris attack suspect Saleh Abdeslam
Belgium has been a top concern for counterterrorism officials for years because of the large number of Belgian foreign fighters who traveled to join ISIS and other terror groups in Syria and Iraq. Many have been returning.
Last Friday, after more than four months on the run, Paris attack suspect Salah Abdeslam was captured after being wounded in a gunfight with Belgium police in Molenbeek. Days later, Belgium and French authorities warned of more attacks.
On March 15, police killed an Algerian, Mohamed Belkaid, during a raid in a Brussels suburb. Prosecutors believe he directed the Paris attackers via calls from Belgium. An ISIS flag was found during the raid.
In January, prosecutors announced they had identified three homes in Belgium used by the Paris attackers. They found bomb-making materials in one of them.
United States: "This is yet another reminder that the world must unite," President Barack Obama said from Cuba, where he's on a historic visit. "We must be together regardless of nationality or race or faith in fighting against the scourge of terrorism."
France security: France is deploying an additional 1,600 police officers around the country, French Interior Minister Bernard Cazeneuve said Tuesday.
U.S. security: At Washington's Union Station, police were pulling random passengers, as they were boarding, to go through luggage security checks. Bomb-sniffing dogs were also out on the tracks doing sweeps.
Authorities in other U.S. cities,
including New York and Chicago, are taking special precautions, such as increased K9 sweeps of subways and additional police patrols.
The FBI is also boosting checks on the whereabouts of suspected extremists who are already being monitored and checking for any intelligence indicating possible threats in the United States, officials say.
Additionally, U.S. national security officials are on guard for possible copycat attacks, though there is no specific indication of any plots in the United States tied to the Brussels attacks, officials said.
Britain security: British police have increased their presence at certain locations, including transport hubs such as London's Heathrow and Gatwick airports, according to Scotland Yard.
NATO security: The alliance is raising its level of alert, it said in a statement.