One person has been detained over terrorist attacks in Brussels, but the prime suspect in the airport bombings remains on the run, as police find a fresh lead in their investigation.

Police took two people in for questioning on Tuesday night, after three deadly blasts at Brussels Airport and Maelbeek metro station on Tuesday morning killed at least 31 and left 270 injured.

One person had been released while the other remained in police custody. Police were still hunting Najim Laachraoui, who was caught on CCTV footage at Brussels Airport with the two suspected bombers.

Images released by police of Brussels bombing suspect Najim Laachraoui.

Police raided an apartment used by the suspects, and uncovered what was described as a "will" written by one of the airport bombers, along with vast quantities of bomb-making materials.

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SUPPLIED The three Brussels Airport bombing suspects were caught on CCTV.

﻿At a press conference overnight, Belgian federal prosecutor Frederick van Leeuw confirmed brothers Ibrahim and Khalid el-Bakraoui﻿ were two of the suspects killed in the attacks.

The brothers had "very heavy" police files, "but no connection to terrorism", the prosecutor said.

Van Leeuw would not confirm the identity of the third airport bomber, widely named as Laachraoui, but said Belgian media reports that he had been arrested were incorrect.

The prosecutor confirmed Ibrahim detonated the first explosion in row 11 of Zaventem Airport's departure terminal at 7.55am on Tuesday. A second blast, 37 seconds later in row two, was detonated by an as-yet unnamed suicide bomber.

The third suspect - believed to be Laachraoui - deposited his bag and departed before the explosions.

This bag contained the biggest charge, but it did not explode until after a bomb disposal team arrived – and "fortunately nobody was injured" in that explosion.

A series of photograph shows Najim Laachraoui, who was previously identified in a false passport as Soufiane Kayal by Belgium Federal Police.

No other weapon was found at the scene, he said, contradicting early reports, and Islamic State claims, of automatic weapons at the airport.

Khalid was responsible for the bombing at the Maelbeek metro station about an hour later, van Leeuw said. That attack happened in the train's second carriage as it was leaving the station.

The attackers took a taxi to the airport from the suburb of Schaerbeek, and police had spoken to the taxi driver, van Leeuw said.

Armed police raided the apartment the men were picked up from, finding 15kg of explosives and bomb-making material, including 150L of acetone, 30L of oxygenated water, detonators, a bag full of bolts and screws, tools and ventilators.

In a rubbish bin in the same street, investigators found a computer containing a message from Ibrahim el-Bakraoui, which was believed to be a will.

In it, he said he suspected "he was being searched for everywhere, didn't feel secure and may end up in a prison cell", van Leeuw said.

Life began to return to normal in Brussels on Wednesday, with some public transport working and cars returning to the European district, but the metro system remained closed and the airport was still shut to travellers.