Belgian Federal Prosecutor Eric Van der Sypt said authorities were especially interested in any people who might have filmed or photographed the man.
He was seen at the airport with two suicide bombers before they died in the March 22 attacks. A subsequent explosion at Brussels' Maelbeek subway station killed another 16 people the same morning.
Photos released by prosecutors showed the "man in hat" leaving the airport on foot, walking to the nearby town of Zaventem and then into Brussels, where all traces of him were reportedly lost.
The appeal for public assistance more than two weeks after the suicide bombings indicated that investigators have hit a standstill. Three bombers, two at the airport and one in the subway, also died in the attacks, which were claimed by Islamic State.
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According to a video reconstitution of the suspect's itinerary presented to reporters, the man left the Brussels Airport terminal at 7:58 a.M. Before two other men he was with in the building exploded suitcases laden with explosives.
He passed by a Sheraton hotel, walked through the town of Zaventem, discarded his jacket, and was seen on video footage at Meiser square in northeastern Brussels at 9:42 a.M. Eight minutes later, his trail vanishes.
Prosecutors asked "people who might have filmed or taken a photograph of the suspect or think they can provide extra information" to call a special telephone number or to email authorities.
Earlier today, the lawyer for Paris attacks suspect Salah Abdeslam said it will take some weeks before his client can be extradited from Belgium to France.
Mary said before Belgian authorities allow Abdeslam to leave they want to question the 26-year-old about another case - a deadly police raid in the Forest neighborhood of Brussels days before his arrest.