EU justice ministers are meeting in Brussels this afternoon for an extraordinary meeting following the Brussels attacks
It had previously been thought only one suicide bomber had been involved in carrying out the deadly strike on the Brussels underground.
But both Belgian and French media are reporting a second attacker is thought to have been involved and may be on the run.
The suspect was captured by surveillance cameras in the metro on Tuesday carrying a large bag alongside Khalid El Bakraoui, who has been identified by prosecutors as a suicide bomber, according to local media.
However, it is not known if the second suspect was killed in the attack.
Khalid's brother, Ibrahim, was one of two suicide attackers who targeted Brussels airport the same day.
It has emerged his accomplice, who also blew himself up, was Najim Laachraoui (24) identified as the bomb maker in the Paris terror attacks.
Belgium has been accused of ignoring warnings that Ibrahim was a "foreign terrorist fighter".
He was caught in June last year at the Turkish-Syrian border and deported to the Netherlands, but was allowed to walk free after the Belgian authorities failed to establish any terror links, Turkey said.
The latest developments come as the manhunt continues for another suspect dressed in a light-coloured jacket and a hat.
Dubbed the "man in white", he was pictured alongside Ibrahim El Bakraoui and Laachraoui, moments before they detonated their suitcase bombs.
Prosecutors have revealed his explosive device, which failed to detonate, was the largest of them all.
Belgian counter-terror officials have raised the prospect that other suspects linked to the attacks may also still be at large.
A fourth victim of the Brussels terrorist attack has been named as more stories emerge of those caught up in the atrocities.
According to local media reports, 51-year-old Fabienne Vansteenkiste had just finished her shift as a baggage handler when two bombs exploded in the airport.
While it will take days for officials to formally identify her body, her family says it is “realistic” about the situation.
At least 31 people were killed in the attacks on the airport and metro station on Tuesday morning, with another 300 injured.
The death of student Leopold Hecht in the metro station bombing was confirmed with "immense sadness" by the rector of Saint-Louis University in the Belgian capital.
Branding Tuesday's atrocities "barbaric acts", Pierre Jadoul said: "There is no word to describe our dismay in the face of this news. All our thoughts go out to his family and loved ones".
Olivier Delespesse also died at the station.
His death was confirmed by the French Community of Belgium, of which he was a member.
Earlier came confirmation that a Peruvian mother of three-year-old twins was one of those killed in the airport attack.
Adelma Tapia Ruiz (37) died while checking in at Zaventem Airport, ahead of a planned trip to visit relatives in New York.
According to reports her husband and two daughters survived the attack.
The girls are said to have gone off to play, with their father in pursuit, moments before the bomb went off.
Her brother, Fernando Tapia, said his sister had lived in Belgium for nine years after marrying a local man.
"She had also planned to come back to Peru this year. Now she can't come back".
Meanwhile, the chief suspect in the Paris attacks, Salah Abdeslam, has appeared in court in Brussels after his arrest last week in the Belgian capital.
His lawyer has claimed he "didn't know" about the Brussels attacks.
Sven Mary also said his client will no longer fight extradition to France and wanted to be returned to his home country as "quickly as possible" to "explain himself".
At least 31 people were killed and 270 injured in the suicide bomb attacks at Zaventem Airport and Maalbeek station on Tuesday morning.
Acting Justice Minister Frances Fitzgerald is meeting her EU counterparts in Brussels later for emergency talks about the bombings.
An EU statement said the meeting is "intended to show solidarity with Belgium and discuss the actual state of play in the fight against terrorism".
Newstalk.com's Shona Murray is in Brussels. She spoke to Newstalk Breakfast earlier about the developments.
She explained: "Reports are saying Najim Laachraoui - a man who we thought had been arrested yesterday - it appears he was the second suicide bomber at Zaventem [Airport]".