BRUSSELS -- Belgium is observing three days of mourning after three bomb blasts in Brussels killed at least 31 people and wounded 270.
Twin explosions at Zaventem airport and another at a metro station on March 22 were claimed by the Islamic State extremist group.
The federal prosecutor said on March 23 that two brothers -- Khalid and Brahim el-Bakraoui -- carried out the suicide bombings at the airport and on the subway. One of them left a will on a computer that was found.
Prosecutor Frederic Van Leeuw said two other suspects captured on CCTV at the airport have yet to be identified.
Hundreds of people gathered in a historic city square overnight, lighting candles and placing flowers in a vigil for the victims. A minute of silence will be held at midday.
Belgian officials have increased security and launched searches around the country after the attacks, which were claimed by the extremist group Islamic State.
Belgium raised its terror alert to its highest level and dispatched 225 extra troops to Brussels following what Belgian Prime Minister Charles Michel called "violent, cowardly" terrorist attacks.
More than 180 people were reported wounded in the attacks, with Health Minister Maggie de Block saying many were in serious condition.
Belgian security forces conducted raids in the Schaerbeek area of the Belgian capital and reportedly found explosives and an Islamic State (IS) flag.
An Islamic State website said its "soldiers of the caliphate" had carried out the attack.
AMAQ, a news agency affiliated with Islamic State extremists, carried the claim of responsibility.
"Islamic State fighters carried out a series of bombings with explosive belts and devices on Tuesday, targeting an airport and a central metro station in the center of the Belgian capital Brussels," it said.
U.S. President Barack Obama condemned the "outrageous attacks against innocent people, and EU leaders expressed anger over what Sweden's prime minister called an "attack against democratic Europe."
Public transport was shut down in Brussels, incoming planes and trains were diverted, and authorities urged residents to "stay where you are." EU personnel were instructed to remain indoors, and flags outside the European Commission flew at half-staff.
EU President Donald Tusk said, "These attacks mark another low by the terrorists in the service of hatred and violence."
Photos posted on the Internet showed gruesome scenes of damage and destruction at the airport.
A security worker who helped carry the bodies of victims outside told Reuters that some of them had "their legs destroyed, as if the bomb came from a piece of luggage" on the floor.
Belgium has been in the spotlight since militants living there helped carry out coordinated attacks that killed 130 people in Paris on November 13.
The March 22 blasts came four days after Salah Abdeslam, the chief surviving suspect in the Paris attacks, was captured following after a shoot-out in Brussels. Belgian security forces had been on alert for any reprisal action.
Brussels Mayor Yvan Mayeur reported that 14 people were killed and 81 wounded at the airport, while others put the death toll there at 11.
One witness said the blasts sparked panic as parts of the building collapsed onto travelers.
"When I reached the arrivals hall downstairs, an entire side with glass panes collapsed, downstairs where the taxis are," he told Belgian television channel RTBF. "It was complete chaos. Some women were falling to the ground and crying. It was hell."
Another witness, Zach Mouzoun, told France's BFM television that the second explosion brought down ceilings and "there was blood everywhere."
A doctor who treated 11 of the victims at the Gasthuisberg hospital in Leuven was quoted by Flemish-language broadcaster VTM as saying their wounds suggested at least one of the bombs contained nails.
Police said a Kalashnikov rifle and an unused explosive were also found at the airport.
The Belga news agency reported that the assailants fired shots and shouted words in Arabic before the explosions.
Passengers were led onto the tarmac and the Belgian crisis center urged people not to approach the airport.
#JeSuisBruxelles -- The Reaction To The Attacks On Social Media
Another explosion then struck the Maalbeek subway station, close to EU institutions, during the morning rush hour.
Brussels Mayor Yvan Mayeur said at least 20 people died in the blast and 106 others were wounded, including 17 people with critical injuries.
A survivor fought tears as she told RTBF television about her ordeal.
"There was a big flash of light and the whole carriage exploded. The windows came down on us," she said. "Everyone threw themselves on the ground."
Brussels resident Joe Cook, who arrived at the station shortly after the blast, told RFE/RL that he saw commuters "in various states of shock."
"Some were stumbling, some were lying down, some were being tended to by passersby and other folk," he said.
EU Budget Commissioner Kristalina Georgieva, who also handles employee and security issues, wrote on Twitter that EU institutions were working together to ensure the security of their staff and urged all EU personnel to "stay home or inside buildings."
Photo Gallery: Brussels Attacks
The new attacks sparked outrage and an outpouring of solidarity from world leaders.
French President Francois Hollande said "the whole of Europe has been hit," urging the continent to take "vital steps in the face of the seriousness of the threat.”
"We are at war," French Prime Minister Manuel Valls said after a crisis meeting called by Hollande. "We have been subjected for the last few months in Europe to acts of war."
Swedish Prime Minister Stefan Lofven said the blasts were an "attack against democratic Europe."
German Chancellor Angela Merkel said the "despicable attacks" should be met with "determination toward the terrorists."
British Prime Minister David Cameron vowed: "We will never let these terrorists win."
Obama, who was visiting Cuba, said the United States stood "in solidarity" with Belgium.
He pledged that Washington will do "whatever is necessary" to help Belgium bring the perpetrators to justice, adding that the attacks were another reminder that "the world must unite" against the "scourge of terrorism."
In Egypt, Sunni Islam's main seat of learning, Al-Azhar, said the attacks "violate the tolerant teachings of Islam." It also urged the world community to confront terrorism.
Kremlin spokesman Dmitry Peskov said that Russian President Vladimir Putin "strongly condemned these barbaric crimes."
However, Russian Foreign Ministry spokeswoman Maria Zakharova said that what she claimed were the West's "double standards" toward terrorists had led to terrorist attacks in Europe.
EU foreign policy chief Federica Mogherini, tearing up at a joint news conference with Jordanian Foreign Minister Nasser Judeh in Amman, said it was a "very sad day" for Europe.
She said it was clear the blasts were attacks that resulted from “radicalization,” and urged leaders in Europe and the Middle East to work together to tackle the problem.
Airports across Europe have tightened security.
France, which remains in a state of emergency after the November 13 attacks, has also reinforced security on its border with Belgium.
Several landmarks around the world -- including Paris's Eiffel Tower, Berlin's Brandenburg Gate, and New York's World Trade Center -- were illuminated in the black, yellow, and red colors of the Belgian flag.
Thousands of people also gathered in Brussels' historic city square, Place de la Bourse, to show support and mourn the victims in the attacks. People surrounded a large chalk message saying, "Brussels I Love You."