The March 22 attacks by Islamic State (IS) on Brussels airport and a metro station killed 32 people.
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 befo re they died in the March 22 attacks.
A subsequent explosion at the city’s 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 we re reportedly lost.
The suspect also wore a white jacket but discarded it at some point, prosecutors said.
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 IS.
According to a video reconstitution of the suspect’s itinerary presented to reporters, the man left the Brussels Airport terminal at 7:58am before two other men he was with in the building detonated 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 north-eastern Brussels at 9:42am.
Eight minutes later, his trail vanished. Belgian authorities are hoping that they or someone finds the discarded light-coloured jacket, saying it could yield precious clues.
Federal prosecutor Thierry Werts also said there had been many people around the hotel when the suspect walked by who may now be overseas, and asked for their assistance as well.
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.
One of the Brussels bombers had previously worked at the European Parliament as a cleaner.
An official from the EU said that, for a month in 2009 and 2010, one of the men worked in the parliament on a student summer job.
Although it was not confirmed which of the men involved in the attacks was being referred to, it has been reported that it was Najim Laachraoui, one of the airport bombers.
The man had been employed through a cleaning company contracted by the European Parliament at the time, a spokesman said.
However, he said that the man did not have a criminal record at the time as the firm had submitted proof of this to the parliament as required under the terms of its contract.
European Parliament spokesman Jaume Duch Guillot said: “The European Parliament confirms that seven and six years ago, one of the perpetrators of the Brussels terrorist attacks worked for a period of one month for a cleaning company which was contracted by the European Parliament at the time.
“As a student, he held a summer holiday job cleaning at the parliament for one month in 2009 and one month in 2010. Those were the only instances he worked at the parliament. As required by the contract, the cleaning firm submitted proof of the absence of a criminal record to the European Parliament.”
Meanwhile, four people suspected of having “enlisted” in IS with the intention to commit terror have been arrested, Danish police said.
All four suspects, arrested in Copenhagen and its suburbs, are suspected of violating Danish laws by joining a terrorist organisation.
Police spokesman Poul Kjeldsen said arms and munitions had been found in one location but it was not immediately clear whether that could be connected to the arrests.
Kjeldsen said he could not exclude the possibility of more arrests as the investigation continues.
Police declined to disclose further details, saying the p rosecution would ask for closed doors at a pre-trial detention hearing today, which would bar the publication of details.
Kjeldsen said the four suspects had been identified through “close co-operation” between Denmark’s intelligence agency and the police in the Danish capital. Justice minister Soren Pind said “it was too early to say” what the suspects’ plans were.
About 60 Danish nationals have returned from Syria where they have fought alongside IS members.