Belgian authorities had prior warning that Brussels attacker was a terrorist
Belgian authorities allowed one of the Brussels killers to walk free - after being warned by Turkish officials that he was a terrorist.
Ibrahim El Bakraoui, one of the suicide bombers who killed 14 people at Brussels airport, was caught in June at the Turkish-Syrian border and then deported to the Netherlands.
Turkish authorities said it warned Belgium and the Netherlands he was a "foreign terrorist fighter".
Turkish president Recep Tayyip Erdogan said that "despite our warnings that this person was a foreign terrorist fighter, Belgium could not establish any links with terrorism", and he was not detained.
Ibrahim El Bakraoui, who is also known as Brahim, and his brother Khalid both died in the coordinated blasts at the airport and metro on Tuesday along with a third suicide bomber, whose identity is not known.
A massive international manhunt has been launched to hunt down the fourth attacker, who has been named in reports as 24 year-old Najim Laachraoui.
Prosecutors said at least 31 people were killed and 270 injured in the attacks.
As Belgians began three days of mourning, the country remained on high alert meaning there was danger of an imminent attack, said Paul Van Tigchelt, head of Belgium's terrorism threat body.
As government offices, schools and residents held a moment of silence to honour the dead, the mood was defiance mixed with anxiety that others involved in the attacks are still at large.
Belgian prosecutor Federal Prosecutor Frederic Van Leeuw identified two of the Brussels attackers as brothers - Ibrahim El Bakraoui, a suicide bomber at the airport, and Khalid El Bakraoui, who targeted the metro.
Investigators raided the Brussels neighbourhood of Schaerbeek after the attacks and found a computer in a rubbish bin on the street including a note from Ibrahim El Bakraoui saying he felt increasingly unsafe and feared landing in prison.
He was the brother who Turkish officials said was deported from Turkey to the Netherlands. Belgium's justice minister said authorities there knew him as a common criminal, not an extremist.
A taxi driver who took Ibrahim El Bakraoui and two others to the airport led investigators to an apartment where they found 15 kilograms (33lbs) of TATP explosives, along with nails and other materials used to make bombs, Mr Van Leeuw said.
He said authorities do not know the identities of two other people pictured with El Bakraoui in a surveillance photo from the airport that police are circulating.
Two were suicide bombers, the prosecutor said; the other was a man in a white jacket and black cap who fled before the bombs went off, leaving behind a bag full of explosives. That bag later blew up, but no one was injured.
The Islamic State group, which was behind the Paris attacks, has also claimed responsibility for the Brussels bombings.
Belgian state broadcaster RTBF, citing sources it did not identify, said Khalid El Bakraoui had rented an apartment that was raided last week in an operation that led authorities to top Paris attacks suspect Salah Abdeslam.
Abdeslam was arrested on Friday in the Brussels neighbourhood where he grew up.
Authorities are also still looking for a suspected accomplice of Abdeslam, Najim Laachraoui, whom they have been searching for since last week. It has not been confirmed if he has any connection to the Brussels attack.
Laachraoui is believed to have made the suicide vests used in the Paris attacks, a French police official told AP, adding that Laachraoui's DNA was found on all of the vests as well as in a Brussels apartment where they were made.
French and Belgian authorities have said in recent days that the network behind the Paris attacks was much larger than initially thought - and developments this week suggest the same group could have staged both the Paris and Brussels attacks.
The airport and several Brussels metro stations remained closed on Wednesday, and authorities said the airport would remain closed at least until the Saturday, forcing the cancellation of 600 flights each day.
Thousands of people have been gathering at Place de la Bourse in the centre of Brussels - including dozens of students chanting "stop the war" - in solidarity with those killed.
Belgians paid homage and lit candles, the mood almost buoyant as people wrote on the ground with big sticks of chalk, drawing peace signs and hearts.
As befits an international city like Brussels, the foreign minister said the dead collectively held at least 40 nationalities.
"It's a war that terrorism has declared not only on France and on Europe, but on the world," French Prime Minister Manuel Valls said on Europe-1 radio. Mr Valls, who planned to visit Brussels later on Wednesday, urged tougher controls of the EU's external borders.
"We must be able to face the extension of radical Islamism ... that spreads in some of our neighbourhoods and perverts our youth," he said.
The Paris attackers were mainly French and Belgian citizens of North African descent, some from uthat struggle with discrimination, unemployment and alienation.
In its claim of responsibility, the Islamic State group said its members detonated suicide vests both at the airport and in the metro.
IS warned of further attacks, issuing a statement promising "dark days" for countries taking part in the US-led anti-IS coalition in Syria and Iraq.
European security officials have been bracing for a major attack for weeks and had warned that IS was actively preparing to strike.
Mr Valls said that big events, be they sports or cultural, must not be put on hold for fear of attacks - that includes the Euro2016 soccer tournament, a month-long event being held in France that starts in June.
Security officials told AP that the Islamic State group has trained at least 400 attackers and sent them into Europe for terror attacks.
The network of interlocking, agile and semi-autonomous cells shows the reach of the extremist group in Europe even as it loses ground in Syria.
The officials, including European and Iraqi intelligence officials and a French lawmaker who follows the jihadi networks, described camps designed specifically to train for attacks against the West.
The officials said the fighters have been given orders to find the right time, place and method to carry out their mission.