A picture emerged Wednesday of the men who bombed the Brussels airport and a central subway line killing scores, with prosecutors and media naming two brothers and a Belgian with links to Paris as the search continued for a fourth attacker.

Brothers Khalid and Ibrahim El Bakraoui were known to police and had extensive criminal records that were not linked to terrorism, federal prosecutor Frederic Van Leeuw said. The third man, reported by state broadcaster RTBF to be Najim Laachraoui, was already sought as a suspected bomb maker in November's terrorist attacks in Paris.

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.

A fourth 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 31 people dead and 300 injured. The Belgian Health Ministry said that 150 people were still hospitalized and 61 were in intensive care.

Most of the injured, representing 40 nationalities including three European Commission staff members, suffered burns that in some cases were accompanied by "war injures like lesions related to a powerful blast," the ministry said.

The Islamic State jihadist group claimed responsibility for the attacks.

Turkish President Recep Tayyip Erdogan said one of the suspects had been detained in his country near the border with Syria, which has long served as a a gateway for fighters, including Islamic extremists, to join armed groups participating in the Syrian civil war.

The man, identified as Ibrahim El Bakraoui by broadcaster CNN Turk, was deported by Turkish officials. Belgian Justice Minister Koen Geens told local media that the man had been deported to the Netherlands.

European officials held a minute of silence to commemorate the victims, as French Prime Minister Manual Valls met with his Belgian counterpart, and US Secretary of State John Kerry's office said he would travel to Brussels on Friday to reiterate US support for the investigation and international efforts to counter violent extremism.

In Brussels, many gathered around an impromptu memorial in front of the former stock exchange wondered how the Belgian-born attackers could have turned on their home.

"I think a whole lot of questions about our society will take place," one of the city's deputy mayors, Ans Persoons, told dpa.

Movement in the city has been limited since the attacks, and the airport will remain closed to passenger flights through Friday. The Maelbeek subway station could remain closed for weeks; some roads in the centre were blocked by police.

Details on the attackers provide potential insight into a complex network reaching across Europe. Laachraoui had been sought for months by his alias Soufiane Kayal in connection to the November 13 attacks at bars, cafes, national stadium and a concert hall in Paris that left 130 people dead.

Laachraoui's false identity was recorded by Hungarian authorities when he travelled to Budapest with the key suspect in the Paris attacks, Salah Abdeslam. Abdeslam was arrested in Brussels last after months on the lam.

Traces of Laachraoui's DNA were also found in a house and an apartment in Belgium that are thought to have been used by the Paris terrorists before their attacks. He travelled to Syria in 2013.

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. He is also thought to have rented a hideout in the southern city of Charleroi that was used to prepare the Paris attacks.

The three airport bombing suspects, including one who is still unidentified, 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, prosecutor 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.

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