Authorities have named three men they believe were responsible for bombings that killed 31 people and wounded 270 others at an airport and on a metro train in Belgium’s capital, Brussels, on Tuesday evening (AEST).

Brothers Khalid and Ibrahim El-Bakraoui detonated bombs at Zaventem airport and in a train carriage in Belgium’s underground metro network, the country’s federal prosecutor Frederic van Leeuw said in a news conference late on Wednesday night (AEST).

Turkey says it tried to warn Belgium that Ibrahim was a foreign fighter sent back to Europe after being arrested on the Turkey-Syria border.

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“We reported the deportation to the Belgian Embassy in Ankara on July 14, 2015, but he was later set free,” Turkish President Tayyip Erdogan said.

It is believed Belgium may have had no option to release El-Bakraoui as no links to crime were found.

Images of the three men pushing luggage trollies towards the check-in counter at the airport were released on Wednesday, as a manhunt was launched for Laachraoui, who is still on the run.

“The third man is on the run; he left his bag with the biggest bomb in it which exploded later because it was so unstable,” Mr Van Leeuw said, referring to the man in a hat and white coat in the CCTV images.

“This third person remains unidentified and is still being looked for,” he said, adding that another man – seen in black on the left of the footage – was also unknown.

The one in the middle was Ibrahim El Bakraoui, whose brother Khalid blew himself up on the Brussels metro, he said.

Zaventem’s mayor said the explosives were stowed in their luggage and detonated before reaching the security gate.

The prosecutor also revealed Ibrahim El Bakraoui left a will saying “I don’t know what to do” in a trash can.

Bakraoui’s will said he was “in a rush, I don’t know what to do, hunted everywhere, no longer safe” and “I don’t want to end up in a cell next to him”, Mr van Leeuw said.

That appeared to be a reference to Paris attacks suspect Salah Abdeslam, who is reportedly linked to Bakraoui, and who is in custody in Belgium after being captured last week.

Blasts in the departure hall of the airport terminal killed 11, and wounded a further 90 people, while at least 20 died and more than 100 injured in a separate bombing at the Maalbeek metro station on Rue de la Loi.

Najim Laachraoui, formerly thought to be named Soufiane Kayal, was been identified as an accomplice of Salah Abdeslam. Photo: AAP

The fatal attacks, for which Islamic State claimed responsibility, were about an hour apart.

Connection to November’s Paris attacks

All three were known to police before the deadly blasts and believed to be connected to Abdeslam.

The El-Bakraoui brothers, both Belgian nationals, were “long-standing criminals”, The Guardian reported, with “clear links” to the Paris attacks.

Laachraoui was identified as a suspect in the November killings on Monday, but before then was only known by the false name, Soufiane Kayal.

Authorities found his DNA on explosive belts used at the Bataclan Theatre and Stade de France and at an apartment used by the Paris assailants in Auvelais, near the central Belgian city of Namur, from where the attacks were planned.

The three were on the run following a shoot-out on March 15

Australian counter-terror expert Greg Barton told The New Daily on Tuesday the attacks were “almost certainly” connected to Abdeslam’s arrest.

“French police were saying what happened in Paris in November was a lot larger than what was expected, so that is the worry — that this might not be the end of the threat,” he said.

Brussels’s Zaventem airport and public transport hubs near the attack zone remained closed on Wednesday.

– with ABC and AAP