A terror suspect is believed to be on the run after the Brussels bombings, which left 34 dead and almost 200 injured.
Belgian police issued an image of the man, one of three seen pushing luggage trolleys through Zaventem airport moments before two bombs exploded.
A third bomb was deactivated at the airport hours after the attack.
In a statement, Belgian police appealed for information about the man, who is wearing a hat and a light-coloured jacket.
"Police are looking to identify this man. He is suspected of having committed the attack at Zaventem on Tuesday March 22.
"If you recognise this individual or have information regarding this attack please call 0800 30 300."
The airport attack was followed by a bomb blast on a Metro train in the city centre as terrorists inflicted a new outrage on a European capital.
The coordinated bomb attacks left a major city in lockdown once again less than five months after Paris was hit by a wave of deadly strikes.
The atrocities on Tuesday, condemned as "blind, violent and cowardly" by Belgian Prime Minister Charles Michel, came after the arrest last week of terror mastermind Salah Abdeslam, who plotted November's massacre in the French capital.
The terror group Islamic State (IS) has claimed responsibility for the Brussels attacks.
Security was being stepped up at major transport hubs around the continent, with British police forces boosting numbers at "key locations" including ports, airports and the rail network.
French Prime Minister Manuel Valls said: "We are at war. In Europe we have been subjected to acts of war for several months."
As the city went into lockdown:
:: Local media reported that 20 people were killed following the blast at Maelbeek metro station, while 14 died in the suicide attack at the airport
:: The number of people injured in both attacks was believed to be 198
:: Two Kalashnikov rifles and an unexploded bomb belt were found at the airport
:: Searches were carried out of a residence believed to be linked to the three suspects
:: Downing Street said one British national is known to have been injured
First news of the attacks came when the airport was rocked by a double blast around 7am UK time. It was blamed on suicide bombers.
The attack on the Metro came about an hour later.
Jef Versele, 40, from Ghent, told the Press Association: "I was on my way to check in and two bombs went off - two explosions.
"I didn't see anything. Everything was coming down. Glassware. It was chaos. It was unbelievable. It was the worst thing."
Footage from inside the airport building showed a scene of devastation with ceiling tiles strewn across the floor and suitcases abandoned.
Images of passengers climbing from a train into a smoke-filled tunnel near Maelbeek station were reminiscent of scenes following the July 7 attacks in London.
Other images showed the injured from the Metro being treated in the street, while at the airport people could be seen fleeing in terror in video footage shot from an airport car park.
As the army was deployed and the transport system was locked down, Mr Michel said: "In this tragic and dark moment for our country, more than ever I call everyone to show calm but also solidarity.
"We are faced with a challenge, a difficult challenge, and we have to face this challenge being united, showing solidarity and being together."
International leaders united in support for Belgium, with David Cameron branding the atrocities "appalling" and US president Barack Obama condemning the "outrageous attacks against innocent people".
The Prime Minister's official spokeswoman said Britain was also sending a team of specialist police officers to the city to assist with the investigation.
"They will be gathering together as much material as possible that can help them with the investigation. It is working on the investigation, trying to work out what happened, who is responsible," the spokeswoman said.