By Alastair Macdonald, Foo Yun Chee and Ingrid Melander.
Belgium's interior and justice ministers have offered their resignations for failing to track an Islamic State militant expelled twice by Turkey who went on to blow himself up at Brussels airport this week.
Brahim El Bakraoui was one of three identified suspected suicide bombers who hit the airport and a metro train, killing at least 31 people and wounding at least 270 on Tuesday in the worst attack in Belgian history.
At least one other man seen with them on airport security cameras is on the run and a fifth suspected bomber filmed in the metro attack may be dead or alive.
Interior Minister Jan Jambon and Justice Minister Koen Geens on Thursday tendered their resignations to Prime Minister Charles Michel, who asked them to stay on.
Turkish officials said Bakraoui, 29, was expelled last July after being arrested near the Syrian border and again in August after he returned to Antalya.
Belgian and Dutch authorities were notified of Turkish suspicions that he was a foreign fighter trying to reach Syria.
At the time, Belgian authorities replied that Bakraoui, who had skipped parole after serving less than half of a nine-year sentence for armed robbery, was a criminal but not a militant.
Dutch justice minister Ard van der Steur said Bakraoui was not registered on alert systems at Schipol airport.
Bakraoui's brother Khalid, 26, killed about 20 people at Maelbeek metro station in the city centre and had also reportedly violated his parole last May, but a Belgian magistrate had released him.
The security lapses in a country that is home to the European Union and NATO have drawn international criticism of an apparent reluctance to tackle Islamist radicals effectively.
Belgium, per capita, has the largest contingent of locals who have fought in Syria.
At the time of the Paris attacks, its security service had fewer than 600 staff.
The government has since raised spending on police and intelligence.
Belgium has lowered its security alert level one notch down from the highest level; but officials did not say what that would mean in terms of security measures that have seen a heavy police and military presence in Brussels.
Investigators are convinced the same jihadist network was involved in the November Paris attacks that killed 130 people.
Islamic State posted a video on social media calling the Brussels blasts a victory and featuring the training of Belgian militants suspected in the Paris attacks.
Public broadcaster VRT said investigators believed Paris attacks suspect Salah Abdeslam, arrested last Friday, probably planned a similar shooting and suicide bomb attack in Brussels.
The lawyer for 26-year-old French national Abdeslam, said he wanted to "explain himself" and would no longer resist extradition to France.
Abdeslam, said lawyer Sven Mary, didn't know about the plan for the Brussels attacks that were carried out by men who had shared hideouts with him.
After calls from US Republican presidential frontrunner Donald Trump for the possible use of torture in such cases, Belgian officials have faced questions over their failure to extract prior intelligence from Abdeslam.
Security sources told Belgian media the other suicide bomber at the airport was Najim Laachraoui, a veteran Belgian Islamist fighter in Syria suspected of making explosive belts for November's Paris attacks.
Laachraoui's younger brother Mourad issued a statement condemning his actions, saying he had had no contact with him since he had left for Syria.
It was the first public reaction from a family member of one of the Brussels attackers.
King Philippe, Prime Minister Michel and European Commission President Jean-Claude Juncker attended a memorial event at parliament.
French Prime Minister Manuel Valls demanded a "strong European response", but officials say many states, including France, withhold their most cherished intelligence despite a mantra of willingness to share information.