Belgium's federal prosecutor named Ibrahim El Bakraoui as one of two men who blew themselves up at Brussels airport Tuesday, while his brother Khalid struck a crowded metro train in coordinated blasts that left 31 dead and 270 injured.
"The third man is on the run," prosecutor Frederic Van Leeuw said of a man seen on CCTV pushing a trolley through Zaventem Airport alongside the two suicide bombers, shortly before the attacks claimed by the Islamic State jihadist group.
Belgian investigators have unleashed a dragnet, releasing CCTV images of the three airport attackers -- with the second suicide bomber and the third man, on the run, still unidentified.
Belgium stood still at noon to mark a minute's silence for the victims of the carnage that left bodies strewn across the airport's departures hall and a train at Maalbeek station, near the European Union's headquarters.
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The attackers' link to Abdeslam -- who told investigators he was planning an attack on Brussels -- has underscored fears about Europe's struggle to combat homegrown terrorism.
Van Leeuw revealed that airport bomber Ibrahim El Bakraoui had left a desperate "will" on a computer that he dumped in a trash can on the street, in which he said he was being "hunted everywhere".
Investigators had found an unexploded bomb, an IS flag and bomb-making materials in an apartment on the same street in the Brussels district of Schaerbeek on Tuesday night.
The find also included 15 kilos (33 pounds) of TATP high explosive, Van Leeuw confirmed.
Broadcaster RTBF said metro bomber Khalid El Bakraoui had
rented an apartment in Brussels last week under a false name where Abdeslam's fingerprints were found.
Three days of national mourning have been declared in a country deeply shocked by the carnage.
King Philippe and Prime Minister Charles Michel led a minute's silence outside the EU headquarters in Brussels, the symbolic capital of Europe that is also home to NATO.
And at the city's Place de la Bourse, defiant applause broke out amongst the large crowd gathered to honour the dead, chanting: "Long live Belgium".
The airport announced it would stay shut through Thursday after the bombings that wrecked its main terminal building.
Leaders across Europe have reacted with outrage to the attacks, with the EU vowing to defend democracy and tolerance but also combat terrorism "with all means necessary."
Landmarks from the Eiffel Tower in Paris to Berlin's Brandenburg Gate lit up in the black, yellow and red of Belgium's national flag in solidarity on Tuesday night, while on social media, thousands shared images of beloved Belgian cartoon character Tintin in tears.
Cities across the continent have scrambled to boost security at airports and other potential targets, while Belgium remains on maximum alert.
Australia's Prime Minister Malcolm Turnbull said Europe had "allowed security to slip", questioning the wisdom of EU's Schengen passport-free zone, while the US warned citizens about the "potential risks" of travelling in Europe.
"I'm a bit afraid, especially for my little brothers," said 18-year-old Dominique Salazar as she took her young siblings to school.
Soldiers were checking passengers' bags and the rush-hour crowds were thinner than usual.
Belgian Foreign Minister Didier Reynders said around 40 nationalities were among the dead and wounded, including citizens of Britain, Colombia, France, Peru and the United States.