Brussels attacks: Belgium admits 'errors' over bomber arrest
Belgium has admitted that it made "errors" relating to one of the Brussels attackers.
Turkey has said it arrested and deported Brahim el-Bakraoui last June, warning Belgium he was a "foreign fighter" - but was "ignored".
The Belgian interior and justice ministers said they had offered to resign over this but added that the prime minister refused to let them.
Police reportedly arrested six people in operations on Thursday evening.
The arrests followed house-to-house searches in the Schaerbeek district of Brussels, media reports say, adding that neither the identities of the suspects nor their connection to the attacks had been established.
Tuesday's suicide attacks in Brussels killed 31 people and injured more than 300.
The health ministry said 121 of the injured were still in hospital, 63 of them in a serious condition.
The attacks have been claimed by so-called Islamic State.
Belgium has now lowered its alert to the second-highest level.
Metro stations will gradually reopen with a heavy security presence, and the airport will reopen no earlier than Monday, officials said.
On Thursday evening French Interior Minister Bernard Cazeneuve said that a major operation had been carried out in the town of Argenteuil near Paris leading to the arrest of a suspected militant.
The militant, of French origin, was in an "advanced stage" of plotting an attack in France, Mr Cazeneuve said, adding that no connection had been made to either the Brussels or the November 2015 Paris attacks.
More about the attacks
Turkey's President Recep Tayyip Erdogan said that one of the Brussels attackers had been caught near Turkey's border with Syria in June 2015, and was deported, at his request, to the Netherlands.
Mr Erdogan said Turkey alerted both the Belgian and Dutch authorities, but "despite our warnings that this person was a foreign terrorist fighter, the Belgian authorities could not identify a link to terrorism".
Turkish officials later confirmed he had been talking about Brahim el-Bakraoui.
Dutch Justice Minister Ard Van der Steur on Thursday confirmed that Bakraoui had arrived from Turkey on 14 July 2015, but said he had a valid Belgian passport, was not on any wanted lists and so the Dutch authorities had no reason to detain him.
Belgian Justice Minister Koen Geens said he had been made aware of the deportation but told public broadcaster VRT: "At that time, he was not known here for terrorism. He was a common law criminal out on parole."
Interior Minister Jan Jambon said he understood why there were questions to be answered over why "we missed the chance to seize him when he was in Turkey".
"In the circumstances it was right to take political responsibility and I offered my resignation to the prime minister," Mr Jambon said.
But he added: "The prime minister and the inner cabinet requested clearly this morning that I stay on, given the current situation, that in a war situation you cannot leave the field."
Later unnamed US officials said Brahim Bakraoui and his brother Khalid were on US counter-terror watch lists before the attacks.
But it was not clear whether the officials meant that they had gone on the list before or after last November's Paris attacks, in which 130 people died and which have been linked to the Brussels attacks.
And FBI Terrorist Screening Center spokesman Dave Joly said the centre would not publicly confirm or deny any individual's presence in its database.
EU interior and justice ministers are meeting in Brussels, where they stressed the need to jointly investigate the networks involved in the Paris and Brussels attacks and "set up a joint liaison team of national counter-terrorism experts" at Europol.
Brahim el-Bakraoui is one of three men - pictured in the middle on a CCTV image of them - who carried out the bombings at Zaventem airport that killed 11 people.
Unconfirmed reports say another of the airport attackers was the wanted jihadist Najim Laachraoui, whose DNA was found on explosives linked to last year's attacks in Paris. The third suspected airport attacker has not been identified yet and is on the run.
Bakraoui's brother Khalid struck at Maelbeek metro station, where 20 people died.
There are reports of a second suspect being sought for that attack. One source told AFP news agency that a man with a large bag had been seen beside Khalid el-Bakraoui on surveillance footage at the metro station.
Meanwhile, VRT reports that investigators are working on the assumption that the cell had been planning a far bigger attack, involving Paris-style shootings as well as suicide bombings.
Links are emerging with Salah Abdeslam, a suspect in the Paris attacks.
Abdeslam was arrested and wounded in a police raid on a flat in the Forest area of Brussels last Friday - four days before the attacks in the Belgian capital.
On Thursday, his lawyer said he had changed his mind and would not fight extradition from Belgium to France.
Abdeslam, a 26-year-old French national born in Belgium, did not have prior knowledge of the Brussels bombings and had stopped co-operating with police following the attacks, his lawyer Sven Mary said.
A court hearing on Thursday on the detention of Abdeslam and two other suspects has been postponed until 7 April.
The director of the EU's police agency, Europol, has told the BBC the network of jihadists in Europe is "more extensive than perhaps we first feared".
Robert Wainwright said there were concerns over "a community of 5,000 suspects that have been radicalised in Europe, that have travelled to Syria and Iraq for conflict experience, some of whom - not all - have since come back to Europe".