The attacker is seen wearing light-colored clothing and a hat.
The death toll from March 22 attacks in Brussels rose to 35 after four people died of their injuries in hospital.
More than 300 people were also injured in the attacks, which were claimed by ISIS.
The death toll does not include three attackers, two of whom blew themselves up at the airport and one in the metro.
Investigators have not commented on reports in the Belgian media that the third airport attacker is Faycal Cheffou, a freelance journalist arrested on March 24 outside the prosecutor’s office.
On March 26, a man named Faycal C was charged with “participation in the activities of a terrorist group, terrorist murders and attempted terrorist murders”, a prosecutor’s statement said.
Separately, three men were charged on March 28 with belonging to a terrorist group.
The three, whose names were given as Yassine A, Mohamed B and Aboubaker O, were arrested during raids on 13 addresses on March 27. A fourth man was released without charge.
Belgian Health Minister Maggie De Block announced the latest deaths in a tweet: “Four patients deceased in hospital. Medical teams did all possible. Total victims: 35. Courage to all the families.”
Of the 35 victims, seven have still to be identified, the Belgian crisis center said on March 28.
At least 12 of the victims are foreign nationals from the US, the Netherlands, Sweden, Germany, France, the UK, Italy and China, it said earlier.
More arrests have also taken place in relation to what authorities say were planned attacks on France.
A man already in Belgian custody was reported to have been charged in connection with a foiled attack in the Paris region.
Separately, Dutch police announced on March 27 that they had detained a 32-year-old Frenchman in Rotterdam at the request of French authorities.
The man was arrested on suspicion of preparing an attack in France and will be extradited to the country. Three other people were also detained.
The Frenchman is allegedly linked to Reda Kriket, who was arrested in a Paris suburb on March 24 and said to be in the “advanced stage” of plotting an attack, AFP news agency reported, citing a police source.
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