Two men in dark tops are suspected of carrying out airport bombings while a third man wearing a hat is on the run, reports say.
A picture has emerged showing three men who could be involved in the deadly bomb attacks at an airport and metro station in Brussels.
A man pictured to the right of them, wearing a white jacket, a hat and also with a trolley, is on the run and being hunted by police, it is reported.
At least 34 people were killed and nearly 200 wounded after the apparently co-ordinated terror attacks on Tuesday morning.
Shots were fired as the perpetrators at the airport reportedly shouted in Arabic before two explosions ripped through the departure hall.
A third bomb, thought to be an unexploded suicide belt which was found at the scene, was later destroyed by authorities.
Terrorists also targeted a train at Maalbeek metro station in a bomb attack about an hour after the airport atrocity.
The Islamic State group has claimed responsibility for the attacks, saying its extremists opened fire in the airport and "several of them" detonated suicide belts.
The attacks come a few days after Salah Abdeslam, a suspect in last November's Paris masscare, was arrested in the Molenbeek area of Brussels after four months on the run.
Since his capture Abdeslam last Friday has been questioned by investigators, and he told them he was planning new operations from Brussels.
Prosecutors said two detonators and a large cache of weapons were found in an apartment from which Abdeslam was thought to have fled from last week before eventually being apprehended.
Belgian authorities fear he had accomplices while on the run who are still at large and could pose a threat.
These others include Najim Laachraoui, a suspected Paris attacks accomplice.
Police found his DNA on explosives used during the Paris assault, a source close to the French investigation told the AFP news agency on Monday.
The 24-year-old left for Syria in 2012 and returned to Europe in September using false documents, travelling with Abdeslam using false documents.
Investigators have admitted they are still "far from solving the puzzle" of the Paris attacks, and that they do not know the “full timeline” for what Abdeslam did in between escaping France and being arrested.