Belgian authorities on Thursday were facing questions about possible security lapses prior to the Brussels terrorist attacks following a media report that two of the suspects had violated their parole and a claim by Turkey that it had warned Belgium.

The controversy comes as investigators are still trying to piece together what happened during the attacks on Tuesday, which left at least 31 people dead and some 300 injured at the international airport and a central subway station in the Belgian capital.

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 news agency 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' "serious" 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.

Turkey, meanwhile, has said that it warned Belgium last year about Ibrahim El Bakraoui being - in President Recep Tayyip Erdogan's words - a "foreign terrorist warrior," after he was detained 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.

"He landed with that plane and as any normal Belgian entered through Schiphol," he told journalists in Brussels after a meeting with his EU counterparts.

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 one airport bomber 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. RTBF said it was unclear whether the unidentified man, who was captured on surveillance camera carrying a large bag, was still alive.

The newspaper La Derniere Heure released what it said was a sketch of the suspect, but prosecutors said in a statement that the image has "no relevance" in their investigation.

The second attacker who died at the airport has not been officially identified, but Belgian media have named him as Najim Laachraoui, who was already 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 surviving suspect, Salah Abdeslam, was captured in Brussels just days before the deadly explosions in the Belgian capital.

Although he was expected to fight an extradition request from France, his lawyer, Sven Mary, said in televised remarks on Thursday that Abdeslam wants to be sent to the country "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 from 4 to 3 to indicate that a threat is no longer imminent.

But normalcy has not yet fully returned to the city. The airport, which is Belgium's international aviation hub, announced Thursday that it would not be able to resume any passenger flights until at least Monday.

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