Two Belgian brothers known to police were among the suicide bombers who carried out deadly terrorist attacks this week on the international airport and subway in Brussels, federal prosecutor Frederic Van Leeuw said Wednesday.

Khalid and Ibrahim El Bakraoui have been identified through their fingerprints.

Ibrahim, 29, was one of two suicide bombers who died at Brussels Airport. Khalid, 27, died in the attack at the Maelbeek subway station, where an explosion tore through the second wagon of a train that was headed away from the neighbourhood housing EU institutions.

The RTBF broadcaster reported on Wednesday evening that the other dead attacker at the Brussels airport is Najim Laachraoui, who had been sought by police for months in connection with last year's Paris terrorist attacks.

A third suspect is on the run after having dropped off at the airport a bag containing "the most significant" explosive charge that had been prepared for the attacks, Van Leeuw said. But the bomb only went off later, once a bomb squad was on the scene.

The two attacks in the Belgian capital, which happened about one hour apart on Tuesday, left at least 31 people dead and 270 injured.

Three European Commission staff members are among the injured, Vice President Kristalina Georgieva said. The EU executive's headquarters are located less than 500 metres from the Maelbeek station.

The Islamic State jihadist group has claimed responsibility for both attacks.

Turkish President Recep Tayyip Erdogan said on Wednesday that one of the suspects in the Brussels attacks had been detained in his country last year in Gaziantep, a southern province near the Syrian border.

Turkey's border with Syria long served as a gateway for fighters, including Islamic extremists, to join armed groups participating in the Syrian civil war.

The man was deported and later released by Belgium "despite our warning" that he was a "foreign terrorist fighter," Erdogan said.

The CNN Turk broadcaster, citing Foreign Ministry sources, reported that the man in question was Ibrahim El Bakraoui.

Van Leeuw said that the El Bakraoui brothers had "serious" criminal records in Belgium, but that they were not linked to terrorism.

Ibrahim's criminal history in Belgium included a nine-year prison sentence in 2010 for having shot at police with a Kalashnikov rifle during a hold-up, the Belga news agency wrote.

Khalid, meanwhile, had been sentenced in early 2011 to five years in prison for carjackings, after having been arrested in possession of Kalashnikov rifles, according to the news agency.

Khalid used a false identity to rent an apartment in the Brussels neighbourhood of Forest where a shootout with police occurred during a terrorism raid last week, the broadcaster RTBF said. That incident later led to the arrest of Paris attack suspect Salah Abdeslam.

Khalid is also thought to have rented a hideout in the southern city of Charleroi that was used to prepare the deadly terrorist attacks in Paris last November, the broadcaster said.

The three airport suspects had been picked up by a taxi driver in the Brussels neighbourhood of Schaerbeek before the attack.

While searching the building where the three men had come from, police found 15 kilogrammes of explosives, 150 litres of acetone, 30 litres of hydrogen peroxide, detonators, a suitcase filled with nails and screws and other bomb-making material, Van Leeuw said.

In a trash bin in the same street, investigators found a computer that contained Ibrahim's will.

He wrote that "he was in haste, not knowing what to do, being sought everywhere, not being secure anymore and that if he drags this out he risks to end up ... in a [prison] cell," Van Leeuw said.

The airport and the Maelbeek subway station both sustained heavy damage during the attacks.

The airport - Belgium's hub for international flights - announced that it will remain closed for passenger flights through Friday.

The Maelbeek station is expected to remain closed for weeks, the news agency quoted Brussels mayor Yvan Mayeur as saying.

Belgium is in the midst of three days of national mourning. A minute of silence to honour the victims was held in Brussels on Wednesday. Another national minute of silence is scheduled for Thursday at 2:30 pm. Rallies were also being organized on social media websites.

US Secretary of State John Kerry will travel to the city on Friday to express condolences "for loss of life" in Wednesday's attacks, his spokesman John Kirby said on Twitter.

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