Six people were taken into custody Thursday evening during police raids in Brussels as Belgian authorities faced questions about possible security lapses prior to the Tuesday's terrorist attacks in the city.
No information about the identity of the six detainees was released. Judicial authorities will decide on Friday whether they should becharged, the state prosecutor's office said.
The raids, which involved special police units, took place in the inner city in the neighbourhoods of Schaerbeck and Jette, the Belga news agency said. Three of the suspects were in a car driving near the building housing the prosecutor's office, the report said.
News of the operations came after French Interior Minister Bernard Cazeneuve said a terrorist attack had been thwarted in Paris with the arrest Thursday of a suspected terrorist whose plans for an attack were in an advanced state.
Cazeneuve announced the arrest in a televised speech, saying an anti-terrorism operation was taking place as he spoke in Argenteuil near Paris. He said the suspect was a French national.
As investigators began piecing together what happened in Tuesday's attacks, which left at least 31 people dead and some 300 injured, media reports said two of the suspects were known to Belgian police and had violated their parole.
Two of the dead attackers have been identified as the Brussels-born brothers Ibrahim and Khalid El Bakraoui, who were 29 and 27 years old respectively. Both were convicted criminals who violated their parole, but had not been re-jailed, the Belga reported.
It quoted Belgian Justice Minister Koen Geens as saying that "it is not easy to bring back to prison people who have not respected the conditions of their release in a flagrant manner."
Belgian federal prosecutors had said on Wednesday that the brothers' criminal history was "not linked to terrorism."
But they disclosed Thursday that Belgium had issued an international arrest warrant for Khalid El Bakraoui on December 11, amid suspicions that he had used a false identity to rent a hideout allegedly used by the terrorists who carried out last year's Paris attacks.
A claim by Turkey, which said it had warned Belgium last year about Ibrahim El Bakraoui, also raised questions about information sharing. It was disclosed that Ibrahim El Bakraoui was detained by Turkey near the Syrian border and expelled.
But Geens said on Thursday that Turkey only informed Belgium about the deportation after El Bakraoui had landed at the Schiphol airport in the Netherlands.
Dutch authorities told their parliament that Turkey did not inform them about the reasons for the deportation and that they had no reason to arrest El Bakraoui since he was not listed in Dutch or international police databases.
Geens and Interior Minister Jan Jambon are expected to further explain the incident to the Belgian parliament on Friday. Both men offered to resign, but Prime Minister Charles Michel did not accept.
Khalid El Bakraoui died on Tuesday while helping carry out the suicide bombing at the Maelbeek subway station. His brother was one of two suicide bombers who died the attack at Brussels Airport.
Investigators are trying to track down another suspect who involved in the aiport bombing who is believed to still be on the run.
State broadcaster RTBF reported Thursday that a second man also likely took part in the attack on the Maelbeek station.
The second attacker who died at the airport has not been officially identified, but Belgian media have named him as Najim Laachraoui, who had been sought in connection with the Paris attacks.
His brother, Mourad Laachraoui, on Thursday condemned the Brussels and Paris attacks, saying that he had not had any contact with Najim since he left for Syria in 2013, Belgian media reported.
The Paris attacks in November left 130 people dead. A key suspect, Salah Abdeslam, was captured in Brussels just days before the deadly explosions in the Belgian capital.
His lawyer, Sven Mary, said in televised remarks on Thursday that Abdeslam wants to be extradited to France "as fast as possible."
According to Belgian media reports, Abdeslam and other accomplices had likely planned to carry out a shooting in Brussels that would have coincided with the suicide bombings.
Belgium is marking three days of national mourning as the investigation continues. A tribute to the victims was held at the parliament on Thursday, along with a national minute of silence.
"We will not give in to barbarism," Michel said at the parliament ceremony. "We will not give in to hatred. We stay together, united."
Belgian authorities on Thursday lowered their country's alert level but normalcy has not yet fully returned to the city. The airport announced that it would not be able to resume any passenger flights until at least Monday.